Correctly report error in for-in start in case of proxies (#4165)

In case of Proxies the "ownKeys" call can return an incompatible
value for a for-in statement. In such cases the error should be
propagated to the user.

Fixes: #4159

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.usz@partner.samsung.com

jerry-core/vm/vm.c

@@ -3737,6 +3737,14 @@ vm_loop (vm_frame_ctx_t *frame_ctx_p) /**< frame context */
 
           if (prop_names_p == NULL)
           {
+#if ENABLED (JERRY_ESNEXT)
+            if (JERRY_UNLIKELY (ECMA_IS_VALUE_ERROR (expr_obj_value)))
+            {
+              result = expr_obj_value;
+              goto error;
+            }
+#endif /* ENABLED (JERRY_ESNEXT) */
+
             /* The collection is already released */
             byte_code_p = byte_code_start_p + branch_offset;
             continue;