MDEV-20593 SIGSEGV in report_json_error_ex (on optimized builds).

When first argument to the JSON_MERGE_PATCH was NULL and second - the
invalid JSON line, the error code was garbage. So it should be set to 0
initially.

mysql-test/r/func_json.result

@@ -898,6 +898,11 @@ NULL
 SELECT JSON_MERGE_PATCH(NULL, '[1,2,3]');
 JSON_MERGE_PATCH(NULL, '[1,2,3]')
 [1, 2, 3]
+SELECT JSON_MERGE_PATCH(NULL, 'a');
+JSON_MERGE_PATCH(NULL, 'a')
+NULL
+Warnings:
+Warning	4038	Syntax error in JSON text in argument 2 to function 'json_merge_patch' at position 1
 SELECT JSON_MERGE_PATCH('{"a":"b"}', NULL, '[1,2,3]', '{"c":null,"d":"e"}');
 JSON_MERGE_PATCH('{"a":"b"}', NULL, '[1,2,3]', '{"c":null,"d":"e"}')
 {"d": "e"}

mysql-test/t/func_json.test

@@ -531,6 +531,7 @@ DROP TABLE merge_t;
 
 SELECT JSON_MERGE_PATCH('{"a":"b"}', NULL, '{"c":"d"}');
 SELECT JSON_MERGE_PATCH(NULL, '[1,2,3]');
+SELECT JSON_MERGE_PATCH(NULL, 'a');
 SELECT JSON_MERGE_PATCH('{"a":"b"}', NULL, '[1,2,3]', '{"c":null,"d":"e"}');
 
 --error ER_WRONG_PARAMCOUNT_TO_NATIVE_FCT

sql/item_jsonfunc.cc

@@ -2429,6 +2429,8 @@ String *Item_func_json_merge_patch::val_str(String *str)
   uint n_arg;
   bool empty_result, merge_to_null;
 
+  /* To report errors properly if some JSON is invalid. */
+  je1.s.error= je2.s.error= 0;
   merge_to_null= args[0]->null_value;
 
   for (n_arg=1; n_arg < arg_count; n_arg++)