Fix clang asan reported leak

Fix a realloc() memory leak which would happen when:
- a previous allocation exists ('ptr' is non-NULL); and
- the new 'size' is zero; and
- a voluntary GC (or GC torture) causes the initial realloc() attempt
  to be bypassed

In this case the slow path would incorrectly assume that it was entered
after realloc() had returned a NULL, which for a zero new 'size' would
mean that 'ptr' was successfully freed and no further action was necessary.
But because the realloc() had actually been bypassed, this would cause the
old 'ptr' to leak.

src-input/duk_heap_memory.c

@@ -178,13 +178,7 @@ DUK_LOCAL DUK_NOINLINE_PERF DUK_COLD void *duk__heap_mem_realloc_slowpath(duk_he
 	/* ptr may be NULL */
 	DUK_ASSERT_DISABLE(newsize >= 0);
 
-	/* Newsize was 0 and realloc() returned NULL, this has the semantics
-	 * of free(oldptr), i.e. memory was successfully freed.
-	 */
-	if (newsize == 0) {
-		DUK_D(DUK_DPRINT("zero size realloc in slow path, return NULL"));
-		return NULL;
-	}
+	/* Unlike for malloc(), zero size NULL result check happens at the call site. */
 
 	DUK_D(DUK_DPRINT("first realloc attempt failed, attempt to gc and retry"));
 
@@ -217,7 +211,7 @@ DUK_LOCAL DUK_NOINLINE_PERF DUK_COLD void *duk__heap_mem_realloc_slowpath(duk_he
 
 		DUK_ASSERT(newsize > 0);
 		res = heap->realloc_func(heap->heap_udata, ptr, newsize);
-		if (res || newsize == 0) {
+		if (res != NULL || newsize == 0) {
 			DUK_D(DUK_DPRINT("duk_heap_mem_realloc() succeeded after gc (pass %ld), alloc size %ld",
 			                 (long) (i + 1),
 			                 (long) newsize));
@@ -240,7 +234,7 @@ DUK_INTERNAL DUK_INLINE_PERF DUK_HOT void *duk_heap_mem_realloc(duk_heap *heap,
 #if defined(DUK_USE_VOLUNTARY_GC)
 	/* Voluntary periodic GC (if enabled). */
 	if (DUK_UNLIKELY(--(heap)->ms_trigger_counter < 0)) {
-		goto slowpath;
+		goto gc_retry;
 	}
 #endif
 
@@ -252,22 +246,22 @@ DUK_INTERNAL DUK_INLINE_PERF DUK_HOT void *duk_heap_mem_realloc(duk_heap *heap,
 		DUK_DDD(DUK_DDDPRINT("gc torture enabled, pretend that first realloc attempt fails"));
 		res = NULL;
 		DUK_UNREF(res);
-		goto slowpath;
+		goto gc_retry;
 	}
 #endif
 
 	res = heap->realloc_func(heap->heap_udata, ptr, newsize);
-	if (DUK_LIKELY(res != NULL)) {
+	if (DUK_LIKELY(res != NULL) || newsize == 0) {
+		if (res != NULL && newsize == 0) {
+			DUK_DD(DUK_DDPRINT("first realloc attempt returned NULL for zero size realloc, accept and return NULL"));
+		}
 		return res;
-	}
-
-slowpath:
-
-	if (newsize == 0) {
-		DUK_D(DUK_DPRINT("first realloc attempt returned NULL for zero size realloc, use slow path to deal with it"));
 	} else {
-		DUK_D(DUK_DPRINT("first realloc attempt failed, attempt to gc and retry"));
+		goto gc_retry;
 	}
+	/* Never here. */
+
+gc_retry:
 	return duk__heap_mem_realloc_slowpath(heap, ptr, newsize);
 }
 
@@ -289,10 +283,7 @@ DUK_LOCAL DUK_NOINLINE_PERF DUK_COLD void *duk__heap_mem_realloc_indirect_slowpa
 	DUK_ASSERT(heap->realloc_func != NULL);
 	DUK_ASSERT_DISABLE(newsize >= 0);
 
-	if (newsize == 0) {
-		DUK_D(DUK_DPRINT("zero size indirect realloc in slow path, return NULL"));
-		return NULL;
-	}
+	/* Unlike for malloc(), zero size NULL result check happens at the call site. */
 
 	DUK_D(DUK_DPRINT("first indirect realloc attempt failed, attempt to gc and retry"));
 
@@ -343,9 +334,8 @@ DUK_LOCAL DUK_NOINLINE_PERF DUK_COLD void *duk__heap_mem_realloc_indirect_slowpa
 		 * The pointer being reallocated may change after every mark-and-sweep.
 		 */
 
-		DUK_ASSERT(newsize > 0);
 		res = heap->realloc_func(heap->heap_udata, cb(heap, ud), newsize);
-		if (res || newsize == 0) {
+		if (res != NULL || newsize == 0) {
 			DUK_D(DUK_DPRINT("duk_heap_mem_realloc_indirect() succeeded after gc (pass %ld), alloc size %ld",
 			                 (long) (i + 1),
 			                 (long) newsize));
@@ -370,7 +360,7 @@ DUK_INTERNAL DUK_INLINE_PERF DUK_HOT void *duk_heap_mem_realloc_indirect(duk_hea
 #if defined(DUK_USE_VOLUNTARY_GC)
 	/* Voluntary periodic GC (if enabled). */
 	if (DUK_UNLIKELY(--(heap)->ms_trigger_counter < 0)) {
-		goto slowpath;
+		goto gc_retry;
 	}
 #endif
 
@@ -382,23 +372,23 @@ DUK_INTERNAL DUK_INLINE_PERF DUK_HOT void *duk_heap_mem_realloc_indirect(duk_hea
 		DUK_DDD(DUK_DDDPRINT("gc torture enabled, pretend that first indirect realloc attempt fails"));
 		res = NULL;
 		DUK_UNREF(res);
-		goto slowpath;
+		goto gc_retry;
 	}
 #endif
 
 	res = heap->realloc_func(heap->heap_udata, cb(heap, ud), newsize);
-	if (DUK_LIKELY(res != NULL)) {
+	if (DUK_LIKELY(res != NULL) || newsize == 0) {
+		if (res != NULL && newsize == 0) {
+			DUK_DD(DUK_DDPRINT(
+			    "first indirect realloc attempt returned NULL for zero size realloc, accept and return NULL"));
+		}
 		return res;
-	}
-
-slowpath:
-
-	if (newsize == 0) {
-		DUK_D(DUK_DPRINT(
-		    "first indirect realloc attempt returned NULL for zero size realloc, use slow path to deal with it"));
 	} else {
-		DUK_D(DUK_DPRINT("first indirect realloc attempt failed, attempt to gc and retry"));
+		goto gc_retry;
 	}
+	/* Never here. */
+
+gc_retry:
 	return duk__heap_mem_realloc_indirect_slowpath(heap, cb, ud, newsize);
 }