Stop using /dev/null to find out the serialized object length

Now the rdbSave* functions return the number of bytes written (or
required to write) in serializing a Redis object, writing to /dev/null
and using ftell (which doesn't work on FreeBSD) isn't needed anymore.

src/redis.c

@@ -814,11 +814,6 @@ void initServer() {
     setupSigSegvAction();
 
     server.mainthread = pthread_self();
-    server.devnull = fopen("/dev/null","w");
-    if (server.devnull == NULL) {
-        redisLog(REDIS_WARNING, "Can't open /dev/null: %s", server.neterr);
-        exit(1);
-    }
     server.clients = listCreate();
     server.slaves = listCreate();
     server.monitors = listCreate();