[PATCH] media: meson: vdec: fix NULL pointer deref in vdec_try_fmt_common

[Christian Hewitt <christianshewitt@gmail.com>]

When VIDIOC_TRY_FMT is called with an unsupported pixel format on the
OUTPUT queue, vdec_try_fmt_common() falls back to V4L2_PIX_FMT_MPEG2.
However, if a distro has locally patched MPEG2 support out (as it has
been broken for some time) the platform format table does not contain
MPEG2 so find_format() returns NULL and the subsequent dereference of
fmt_out->max_width triggers a NULL pointer dereference.

Fix this by falling back to the first format in the platform's format
array instead of hardcoding V4L2_PIX_FMT_MPEG2. This is always valid
since every platform defines at least one format.

Fixes: 3e7f51bd9607 ("media: meson: add v4l2 m2m video decoder driver")
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>

---
This fixes use of v4l2-ctl and v4l2-compliance tools when MPEG2 support
has been disabled via vdec_platform.c - NB: although this is a fix it's
also a rather niche scenario so there's no need to backport it to older
stable kernels.

 drivers/staging/media/meson/vdec/vdec.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/media/meson/vdec/vdec.c b/drivers/staging/media/meson/vdec/vdec.c
index 4b77ec1af5a7..f3e7072f221a 100644
--- a/drivers/staging/media/meson/vdec/vdec.c
+++ b/drivers/staging/media/meson/vdec/vdec.c
@@ -504,8 +504,8 @@ vdec_try_fmt_common(struct amvdec_session *sess, u32 size,
 	case V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE:
 		fmt_out = find_format(fmts, size, pixmp->pixelformat);
 		if (!fmt_out) {
-			pixmp->pixelformat = V4L2_PIX_FMT_MPEG2;
-			fmt_out = find_format(fmts, size, pixmp->pixelformat);
+			pixmp->pixelformat = fmts[0].pixfmt;
+			fmt_out = &fmts[0];
 		}
 		break;
 	case V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE:
-- 
2.43.0