CID 452208: locking issue in tests/bench/qpmulti.c

Coverity Scan claims a locking issue in a test code rooted in !7582 (merged).

/tests/bench/qpmulti.c: 296 in read_transactions()
290     			if (result == ISC_R_SUCCESS) {
291     				args->present++;
292     			} else {
293     				args->absent++;
294     			}
295     		}
>>>     CID 452208:  API usage errors  (LOCK)
>>>     "dns_qpread_destroy" unlocks "args->multi->mutex" while it is unlocked.
296     		dns_qpread_destroy(args->multi, &qp);
297     	}
298     	next_loop(args, start);
299     }
300     
301     static void