CVE-2026-93236: media: meson: vdec: fix NULL pointer deref in vdec_try_fmt_common

In the Linux kernel, the following vulnerability has been resolved: media: meson: vdec: fix NULL pointer deref in vdec_try_fmt_common 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.