mm-video: vidc: Avoid setting invalid timestamp of eos NAL

In arbitrary mode when eos NAL is appened at the end, while
coalscing with destination buffer, the actual timestamp is
overwritten with LLONG_MAX. This will lead to error log and
setting the flag V4L2_QCOM_BUF_TIMESTAMP_INVALID in driver,
which is not needed.

Change-Id: I87a63c4cb9d8259280b781cdd553050580809e8e