Fix clang analyzer warning

redisAsyncInitialize() can return NULL, but then we pass
the return value from redisAsyncInitialize() into something
dereferencing the return value, which can cause crashies.

async.c

@@ -142,6 +142,9 @@ static redisAsyncContext *redisAsyncInitialize(redisContext *c) {
 /* We want the error field to be accessible directly instead of requiring
  * an indirection to the redisContext struct. */
 static void __redisAsyncCopyError(redisAsyncContext *ac) {
+    if (!ac)
+        return;
+
     redisContext *c = &(ac->c);
     ac->err = c->err;
     ac->errstr = c->errstr;