Invert two sides of if expression in SET to avoid a lookup.

Because of the short circuit behavior of && inverting the two sides of
the if expression avoids an hash table lookup if the non-EX variant of
SET is called.

Thanks to Weibin Yao (@yaoweibin on github) for spotting this.

src/t_string.c

@@ -26,7 +26,7 @@ void setGenericCommand(redisClient *c, int nx, robj *key, robj *val, robj *expir
         if (unit == UNIT_SECONDS) milliseconds *= 1000;
     }
 
-    if (lookupKeyWrite(c->db,key) != NULL && nx) {
+    if (nx && lookupKeyWrite(c->db,key) != NULL) {
         addReply(c,shared.czero);
         return;
     }