MDBF-534: Coverity scan: fix client folder

```------------------------------
File: `mysqlcheck`
```

------------------------------

- Coverity (FORWARD_NULL):
https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728409&defectInstanceId=53075052&mergedDefectId=1520314&eventId=53075052-7

`mysqlcheck` - make sure `op` is non-null

- Coverity (TAINTED_SCALAR) - FALSE POSITIVES:
https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728409&defectInstanceId=53074482&mergedDefectId=1519904

client/mysqlcheck.c

@@ -942,6 +942,7 @@ static int handle_request_for_tables(char *tables, size_t length,
     DBUG_RETURN(1);
   if (dont_quote)
   {
+    DBUG_ASSERT(op);
     DBUG_ASSERT(strlen(op)+strlen(tables)+strlen(options)+8+1 <= query_size);
 
     /* No backticks here as we added them before */