MDEV-16054 simple json functions flatline cpu on garbage input.

Incorrect char sentence should be handled properly.

mysql-test/r/func_json.result

@@ -760,3 +760,9 @@ t1	CREATE TABLE `t1` (
   `c2` varchar(38) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
+#
+# MDEV-16054 simple json functions flatline cpu on garbage input.
+#
+select json_array(1,uuid(),compress(5.140264e+307));
+json_array(1,uuid(),compress(5.140264e+307))
+NULL

mysql-test/t/func_json.test

@@ -422,3 +422,10 @@ CREATE TABLE t1 AS SELECT
   JSON_QUOTE(_utf8'foo') AS c2;
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-16054 simple json functions flatline cpu on garbage input.
+--echo #
+
+select json_array(1,uuid(),compress(5.140264e+307));
+

strings/json_lib.c

@@ -1645,6 +1645,8 @@ int json_escape(CHARSET_INFO *str_cs,
         return -1;
       }
     }
+    else /* c_len == 0, an illegal symbol. */
+      return -1;
   }
 
   return (int)(json - json_start);