roqvideoenc: set enc->avctx in roq_encode_init

So far it is only set in roq_encode_frame, but it is used in
roq_encode_end to free the coded_frame. This currently segfaults if
roq_encode_frame is not called between roq_encode_init and
roq_encode_end.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/roqvideoenc.c

@@ -999,6 +999,8 @@ static av_cold int roq_encode_init(AVCodecContext *avctx)
 
     av_lfg_init(&enc->randctx, 1);
 
+    enc->avctx = avctx;
+
     enc->framesSinceKeyframe = 0;
     if ((avctx->width & 0xf) || (avctx->height & 0xf)) {
         av_log(avctx, AV_LOG_ERROR, "Dimensions must be divisible by 16\n");