Misc 'no return value' fixes for unreachable code

src/duk_api_stack.c

@@ -106,9 +106,9 @@ DUK_LOCAL duk_int_t duk__api_coerce_d2i(duk_context *ctx, duk_idx_t index, duk_b
 
 	if (require) {
 		DUK_ERROR(thr, DUK_ERR_TYPE_ERROR, DUK_STR_NOT_NUMBER);
-	} else {
-		return 0;
+		/* not reachable */
 	}
+	return 0;
 }
 
 DUK_LOCAL duk_uint_t duk__api_coerce_d2ui(duk_context *ctx, duk_idx_t index, duk_bool_t require) {
@@ -163,9 +163,9 @@ DUK_LOCAL duk_uint_t duk__api_coerce_d2ui(duk_context *ctx, duk_idx_t index, duk
 
 	if (require) {
 		DUK_ERROR(thr, DUK_ERR_TYPE_ERROR, DUK_STR_NOT_NUMBER);
-	} else {
-		return 0;
+		/* not reachable */
 	}
+	return 0;
 }
 
 /*

src/duk_bi_buffer.c

@@ -171,6 +171,7 @@ DUK_LOCAL duk_hbufferobject *duk__require_bufobj_value(duk_context *ctx, duk_idx
 	}
 
 	DUK_ERROR(thr, DUK_ERR_TYPE_ERROR, DUK_STR_NOT_BUFFER);
+	return NULL;  /* not reachable */
 }
 #endif  /* DUK_USE_BUFFEROBJECT_SUPPORT */