ff_end_tag: assert that the start is aligned.

If its start is not aligned then aligning its end will
likely break many demuxers as they check the size and not
the position.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riff.c

@@ -427,6 +427,8 @@ void ff_end_tag(AVIOContext *pb, int64_t start)
 {
     int64_t pos;
 
+    av_assert0((start&1) == 0);
+
     pos = avio_tell(pb);
     if (pos & 1)
         avio_w8(pb, 0);