avcodec/h261dec: Initialize IDCT context during init

Before 998c9f15d1, initializing an
MpegEncContext's IDCT parts occured in ff_mpv_common_init() and this
has been called in h261_decode_frame(), not h261_decode_init().

Yet said commit factored this out of ff_mpv_common_init() and therefore
there is no reason any more not to set this during init as this commit
does.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h261dec.c

@@ -79,6 +79,7 @@ static av_cold int h261_decode_init(AVCodecContext *avctx)
     avctx->pix_fmt = AV_PIX_FMT_YUV420P;
 
     h->gob_start_code_skipped = 0;
+    ff_mpv_idct_init(s);
 
     ff_thread_once(&init_static_once, h261_decode_init_static);
 
@@ -595,10 +596,6 @@ static int h261_decode_frame(AVCodecContext *avctx, void *data,
 retry:
     init_get_bits(&s->gb, buf, buf_size * 8);
 
-    if (!s->context_initialized)
-        // we need the IDCT permutation for reading a custom matrix
-        ff_mpv_idct_init(s);
-
     ret = h261_decode_picture_header(h);
 
     /* skip if the header was thrashed */