avcodec/c93: force dimensions to the correct value instead of depending on the demuxer to do so

Fixes Ticket2934
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/c93.c

@@ -127,6 +127,8 @@ static int decode_frame(AVCodecContext *avctx, void *data,
     uint8_t *out;
     int stride, ret, i, x, y, b, bt = 0;
 
+    avcodec_set_dimensions(avctx, WIDTH, HEIGHT);
+
     c93->currentpic ^= 1;
 
     if ((ret = ff_reget_buffer(avctx, newpic)) < 0)