aacenc: Fix identification padding when the bitstream is already aligned.

libavcodec/aacenc.c

@@ -464,7 +464,7 @@ static void put_bitstream_info(AVCodecContext *avctx, AACEncContext *s,
     if (namelen >= 15)
         put_bits(&s->pb, 8, namelen - 14);
     put_bits(&s->pb, 4, 0); //extension type - filler
-    padbits = 8 - (put_bits_count(&s->pb) & 7);
+    padbits = -put_bits_count(&s->pb) & 7;
     avpriv_align_put_bits(&s->pb);
     for (i = 0; i < namelen - 2; i++)
         put_bits(&s->pb, 8, name[i]);