Reset op_sec_last_sample_ops when reset requested

This value needs to be set to zero (in addition to
stat_numcommands) or else people may see
a negative operations per second count after they
run CONFIG RESETSTAT.

Fixes antirez/redis#1577

src/config.c

@@ -1763,6 +1763,7 @@ void configCommand(redisClient *c) {
         server.stat_keyspace_hits = 0;
         server.stat_keyspace_misses = 0;
         server.stat_numcommands = 0;
+        server.ops_sec_last_sample_ops = 0;
         server.stat_numconnections = 0;
         server.stat_expiredkeys = 0;
         server.stat_rejected_conn = 0;