Avoid dereferencing NULL in pool_on_disconnect

pool_on_disconnect was assuming a pool object was attached and logging
using its server object. It also checked for NULL, but too late.

src/pool.c

@@ -96,6 +96,11 @@ pool_on_disconnect(const redisAsyncContext *ac, int status) {
 
 	struct pool *p = ac->data;
 	int i = 0;
+
+	if(p == NULL) { /* no need to clean anything here. */
+		return;
+	}
+
 	if (status != REDIS_OK) {
 		char format[] = "Error disconnecting: %s";
 		size_t msg_sz = sizeof(format) - 2 + ((ac && ac->errstr) ? strlen(ac->errstr) : 6);
@@ -107,10 +112,6 @@ pool_on_disconnect(const redisAsyncContext *ac, int status) {
 		}
 	}
 
-	if(p == NULL) { /* no need to clean anything here. */
-		return;
-	}
-
 	/* remove from the pool */
 	for(i = 0; i < p->count; ++i) {
 		if(p->ac[i] == ac) {