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>
(cherry picked from commit cf82c426fadf90105e1fb9d5ecd267cc3aa2b288)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
diff --git a/libavcodec/roqvideoenc.c b/libavcodec/roqvideoenc.c
index a3c9987..4b00878 100644
--- a/libavcodec/roqvideoenc.c
+++ b/libavcodec/roqvideoenc.c
@@ -943,6 +943,8 @@
 
     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");