bugfix: replace lastcmd with cmd when rewrite BRPOPLPUSH as RPOPLPUSH

There are two problems if we use lastcmd:

1. BRPOPLPUSH cannot be rewrited as RPOPLPUSH in multi/exec
    In mulit/exec context, the lastcmd is exec.
2. Redis will crash when execute RPOPLPUSH loading from AOF
    In fakeClient, the lastcmd is NULL.

src/t_list.c

@@ -596,7 +596,7 @@ void rpoplpushCommand(client *c) {
         signalModifiedKey(c->db,touchedkey);
         decrRefCount(touchedkey);
         server.dirty++;
-        if (c->lastcmd->proc == brpoplpushCommand) {
+        if (c->cmd->proc == brpoplpushCommand) {
             rewriteClientCommandVector(c,3,shared.rpoplpush,c->argv[1],c->argv[2]);
         }
     }