lavf/swfdec: fix flushing with compressed swf.

We now only return in the middle of the refill in case of read error, so
inflate can be re-called if zlib needs an empty (z.avail_in=0) inflate
call for flushing.

libavformat/swfdec.c

@@ -72,7 +72,7 @@ static int zlib_refill(void *opaque, uint8_t *buf, int buf_size)
 retry:
     if (!z->avail_in) {
         int n = avio_read(s->pb, swf->zbuf_in, ZBUF_SIZE);
-        if (n <= 0)
+        if (n < 0)
             return n;
         z->next_in  = swf->zbuf_in;
         z->avail_in = n;