avcodec: Don't assume separate u and v planes in ff_alloc_picture

alloc_frame_buffer in ff_alloc_picture asserts that the linesize
of planes 1 and 2 are the same. If the pixfmt has a single uv
plane, like NV12, this won't be true.

So, let's only do this check if there are more than 2 planes.

We never hit this with previous hw formats because they don't set
linesize to meaningful values, but the cuda hw format sets the
values based on the underlying data layout.

libavcodec/mpegpicture.c

@@ -22,6 +22,7 @@
 
 #include "libavutil/avassert.h"
 #include "libavutil/common.h"
+#include "libavutil/pixdesc.h"
 
 #include "avcodec.h"
 #include "motion_est.h"
@@ -155,7 +156,8 @@ static int alloc_frame_buffer(AVCodecContext *avctx,  Picture *pic,
         return -1;
     }
 
-    if (pic->f->linesize[1] != pic->f->linesize[2]) {
+    if (av_pix_fmt_count_planes(pic->f->format) > 2 &&
+        pic->f->linesize[1] != pic->f->linesize[2]) {
         av_log(avctx, AV_LOG_ERROR,
                "get_buffer() failed (uv stride mismatch)\n");
         ff_mpeg_unref_picture(avctx, pic);