CVE-2023-52888 media: mediatek: vcodec: Only free buffer VA that is not NULL

In the Linux kernel, the following vulnerability has been resolved:

media: mediatek: vcodec: Only free buffer VA that is not NULL

In the MediaTek vcodec driver, while mtk_vcodec_mem_free() is mostly
called only when the buffer to free exists, there are some instances
that didn’t do the check and triggered warnings in practice.

We believe those checks were forgotten unintentionally. Add the checks
back to fix the warnings.