mp4: Fail early on invalid sample rate or sample count.

mp4: Fail early on invalid sample rate or sample count.

If the sample rate or the sample count happen to be zero, we should
fail the open rather than return success and let the caller deal with
it. This patch moves the corresponding sanity checks from aac_afh.c
to mp4_open_read() of mp4.c. The sample rate is always read while
sample count is skipped for metadata-only opens. So the first check
belongs to the common open_file() while the second check needs to go
to mp4_open_read().