Cleaner fix for Detaching notify write_cb issue

NULL all debugger transport I/O callbacks (i.e. callbacks other than
detaching_cb) whenever the transport indicates an error has occurred.
By the API contract we're not allowed to call any of them after transport
read/write error so it's safest to NULL them all on a transport error.

This is not strictly necessary because only the write_cb is used by
detach1() currently before NULLing the callbacks anyway.  However, NULLing
them early on transport error ensures there's no potential to call them
incorrectly even when the code is reorganized later.

src/duk_debugger.c

@@ -112,6 +112,22 @@ DUK_INTERNAL void duk_debug_do_detach(duk_heap *heap) {
 	duk__debug_do_detach2(heap);
 }
 
+/* Called on a read/write error: NULL all transport related callbacks except
+ * the detached callback so that we never accidentally call them after a
+ * read/write error has been indicated.
+ */
+DUK_LOCAL void duk__debug_null_io_callbacks(duk_hthread *thr) {
+	duk_heap *heap;
+	heap = thr->heap;
+	DUK_D(DUK_DPRINT("transport read/write error, NULL all I/O callbacks (everything but detached)"));
+	heap->dbg_read_cb = NULL;
+	heap->dbg_write_cb = NULL;  /* this is especially critical to avoid another write call in detach1() */
+	heap->dbg_peek_cb = NULL;
+	heap->dbg_read_flush_cb = NULL;
+	heap->dbg_write_flush_cb = NULL;
+	/* keep heap->dbg_detached_cb */
+}
+
 /*
  *  Debug connection peek and flush primitives
  */
@@ -250,8 +266,8 @@ DUK_INTERNAL void duk_debug_read_bytes(duk_hthread *thr, duk_uint8_t *data, duk_
 #endif
 		got = heap->dbg_read_cb(heap->dbg_udata, (char *) p, left);
 		if (got == 0 || got > left) {
-			heap->dbg_write_cb = NULL;  /* squelch further writes in detach1() */
 			DUK_D(DUK_DPRINT("connection error during read, return zero data"));
+			duk__debug_null_io_callbacks(thr);  /* avoid calling write callback in detach1() */
 			DUK__SET_CONN_BROKEN(thr, 1);
 			goto fail;
 		}
@@ -545,7 +561,7 @@ DUK_INTERNAL void duk_debug_write_bytes(duk_hthread *thr, const duk_uint8_t *dat
 #endif
 		got = heap->dbg_write_cb(heap->dbg_udata, (const char *) p, left);
 		if (got == 0 || got > left) {
-			heap->dbg_write_cb = NULL;  /* squelch further writes in detach1() */
+			duk__debug_null_io_callbacks(thr);  /* avoid calling write callback in detach1() */
 			DUK_D(DUK_DPRINT("connection error during write"));
 			DUK__SET_CONN_BROKEN(thr, 1);
 			return;