Fix overflow in mstime() in redis-cli and benchmark.

The problem does not exist in the Redis server implementation of mstime()
but is only limited to redis-cli and redis-benchmark.

Thix fixes issue #839.

src/redis-benchmark.c

@@ -107,7 +107,7 @@ static long long mstime(void) {
     long long mst;
 
     gettimeofday(&tv, NULL);
-    mst = ((long)tv.tv_sec)*1000;
+    mst = ((long long)tv.tv_sec)*1000;
     mst += tv.tv_usec/1000;
     return mst;
 }