shorten: set invalid channels count to 0

Prevent the loop shorten_decode_close from writing and freeing out of
the array boundary.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/shorten.c

@@ -341,6 +341,7 @@ static int read_header(ShortenContext *s)
     s->channels = get_uint(s, CHANSIZE);
     if (s->channels <= 0 || s->channels > MAX_CHANNELS) {
         av_log(s->avctx, AV_LOG_ERROR, "too many channels: %d\n", s->channels);
+        s->channels = 0;
         return -1;
     }
     s->avctx->channels = s->channels;