MDEV-23941: strings/json_lib.c:893:12: style: Suspicious condition

The characters parsed are always ascii characters, hence one byte. This
means that the code did not have "incorrect" logic because the boolean
condition, if true, would also evaluate to the value of 1.

The condition however is semantically wrong, assuming a length is equal
to the condition outcome. Change paranthesis to make it also read
according to the intent.

strings/json_lib.c

@@ -890,7 +890,7 @@ int json_read_keyname_chr(json_engine_t *j)
     case S_QUOTE:
       for (;;)  /* Skip spaces until ':'. */
       {
-        if ((c_len= json_next_char(&j->s) > 0))
+        if ((c_len= json_next_char(&j->s)) > 0)
         {
           if (j->s.c_next == ':')
           {