MDEV-13138 JSON_OBJECT returns null with strings containing backticks.

        Condition was fixed.

mysql-test/r/func_json.result

@@ -674,3 +674,9 @@ JSON_EXTRACT('{"foo": "bar" foobar foo invalid ', '$.foo')
 NULL
 Warnings:
 Warning	4038	Syntax error in JSON text in argument 1 to function 'json_extract' at position 15
+SELECT JSON_OBJECT('foo', '`');
+JSON_OBJECT('foo', '`')
+{"foo": "`"}
+SELECT JSON_OBJECT("foo", "bar`bar");
+JSON_OBJECT("foo", "bar`bar")
+{"foo": "bar`bar"}

mysql-test/t/func_json.test

@@ -322,3 +322,9 @@ select JSON_EXTRACT('{\"input1\":\"\\u00f6\"}', '$.\"input1\"');
 #
 select JSON_EXTRACT('{"foo": "bar" foobar foo invalid ', '$.foo');
 
+#
+# MDEV-13138 JSON_OBJECT returns null with strings containing backticks.
+#
+SELECT JSON_OBJECT('foo', '`');
+SELECT JSON_OBJECT("foo", "bar`bar");
+

strings/json_lib.c

@@ -1586,7 +1586,7 @@ int json_escape(CHARSET_INFO *str_cs,
       enum json_esc_char_classes c_class;
       
       str+= c_len;
-      if (c_chr > 0x60 || (c_class= json_escape_chr_map[c_chr]) == ESC_)
+      if (c_chr >= 0x60 || (c_class= json_escape_chr_map[c_chr]) == ESC_)
       {
         if ((c_len= json_cs->cset->wc_mb(json_cs, c_chr, json, json_end)) > 0)
         {