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

First character of the key name is just skipped, so the escapement
wasn't handled properly.

mysql-test/main/func_json.result

@@ -951,5 +951,17 @@ SELECT json_valid('{"test": "\\ud83d\\ude0b"}');
 json_valid('{"test": "\\ud83d\\ude0b"}')
 1
 #
+# MDEV-19670 json escaped unicode parse error
+#
+SELECT JSON_VALID('{"admin\\"": null}'), '{"admin\\"": null}'
+    UNION
+SELECT JSON_VALID('{"\\"admin": null}'), '{"\\"admin": null}'
+      UNION
+SELECT JSON_VALID('{"\\"": null}'), '{"\\"": null}';
+JSON_VALID('{"admin\\"": null}')	{"admin\"": null}
+1	{"admin\"": null}
+1	{"\"admin": null}
+1	{"\"": null}
+#
 # End of 10.3 tests
 #

mysql-test/main/func_json.test

@@ -568,6 +568,16 @@ SELECT
 SELECT json_valid('{"value":"\\ud83d\\ude0a"}');
 SELECT json_valid('{"test": "\\ud83d\\ude0b"}');
 
+--echo #
+--echo # MDEV-19670 json escaped unicode parse error
+--echo #
+
+SELECT JSON_VALID('{"admin\\"": null}'), '{"admin\\"": null}'
+    UNION
+    SELECT JSON_VALID('{"\\"admin": null}'), '{"\\"admin": null}'
+      UNION
+      SELECT JSON_VALID('{"\\"": null}'), '{"\\"": null}';
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

strings/json_lib.c

@@ -825,6 +825,8 @@ static int skip_colon(json_engine_t *j)
 static int skip_key(json_engine_t *j)
 {
   int t_next, c_len;
+
+  j->s.c_str-= j->sav_c_len;
   while (json_read_keyname_chr(j) == 0) {}
 
   if (j->s.error)