MDEV-19628 JSON with starting double quotes key is not valid.

Make the skip_key a bit faster.

Author: Alexey Botchkov

strings/json_lib.c

@@ -826,7 +826,10 @@ static int skip_key(json_engine_t *j)
 {
   int t_next, c_len;
 
-  j->s.c_str-= j->sav_c_len;
+  if (json_instr_chr_map[j->s.c_next] == S_BKSL &&
+      json_handle_esc(&j->s))
+    return 1;
+
   while (json_read_keyname_chr(j) == 0) {}
 
   if (j->s.error)