avcodec/v4l2_m2m_dec: Fix decoding on Odroid XU4

c0c7946196 unintentianally changed the initialization flow of the
decoder: It caused the capture buffers to be initialized on
v4l2_m2m.c:180 in v4l2_configure_contexts(). This breaks h264 decoding
on the Odroid XU4 (RPI4 was not affected).

This commit postpones capture buffer initialization
as before c0c7946196 to fix the issue.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/v4l2_m2m_dec.c

@@ -201,6 +201,7 @@ static av_cold int v4l2_decode_init(AVCodecContext *avctx)
     capture->av_codec_id = AV_CODEC_ID_RAWVIDEO;
     capture->av_pix_fmt = avctx->pix_fmt;
 
+    s->avctx = avctx;
     ret = ff_v4l2_m2m_codec_init(priv);
     if (ret) {
         av_log(avctx, AV_LOG_ERROR, "can't configure decoder\n");
@@ -209,7 +210,6 @@ static av_cold int v4l2_decode_init(AVCodecContext *avctx)
 
         return ret;
     }
-    s->avctx = avctx;
 
     return v4l2_prepare_decoder(s);
 }