fraps: do not set avctx->pix_fmt to none in decode_init()

This is required for setting up the filterchain, as it requires the
pixel format to be set, which is usually done when probing the file
with av_find_stream_info().

Fix trac issue #83.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

libavcodec/fraps.c

@@ -61,7 +61,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
     FrapsContext * const s = avctx->priv_data;
 
     avctx->coded_frame = (AVFrame*)&s->frame;
-    avctx->pix_fmt= PIX_FMT_NONE; /* set in decode_frame */
 
     s->avctx = avctx;
     s->tmpbuf = NULL;