Fix AOF bug: expire could be removed from key on AOF rewrite.

There was a race condition in the AOF rewrite code that, with bad enough
timing, could cause a volatile key just about to expire to be turned
into a non-volatile key. The bug was never reported to cause actualy
issues, but was found analytically by an user in the Redis mailing list:

https://groups.google.com/forum/?fromgroups=#!topic/redis-db/Kvh2FAGK4Uk

This commit fixes issue #1079.

src/aof.c

@@ -884,6 +884,9 @@ int rewriteAppendOnlyFile(char *filename) {
 
             expiretime = getExpire(db,&key);
 
+            /* If this key is already expired skip it */
+            if (expiretime != -1 && expiretime < now) continue;
+
             /* Save the key and associated value */
             if (o->type == REDIS_STRING) {
                 /* Emit a SET command */
@@ -906,8 +909,6 @@ int rewriteAppendOnlyFile(char *filename) {
             /* Save the expire time */
             if (expiretime != -1) {
                 char cmd[]="*3\r\n$9\r\nPEXPIREAT\r\n";
-                /* If this key is already expired skip it */
-                if (expiretime < now) continue;
                 if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                 if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
                 if (rioWriteBulkLongLong(&aof,expiretime) == 0) goto werr;