Clear server.shutdown_asap on failed shutdown.

When a SIGTERM is received Redis schedules a shutdown. However if it
fails to perform the shutdown it must be clear the shutdown_asap flag
otehrwise it will try again and again possibly making the server
unusable.

src/redis.c

@@ -850,6 +850,7 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
     if (server.shutdown_asap) {
         if (prepareForShutdown(0) == REDIS_OK) exit(0);
         redisLog(REDIS_WARNING,"SIGTERM received but errors trying to shut down the server, check the logs for more information");
+        server.shutdown_asap = 0;
     }
 
     /* Show some info about non-empty databases */