DO NOT MERGE mm-video-v4l2: venc: add safety checks for freeing buffers

Allow only up to 64 buffers on input/output port (since the
allocation bitmap is only 64-wide).
Add safety checks to free only as many buffers were allocated.

Fixes: Heap Overflow and Possible Local Privilege Escalation in
MediaServer (libOmxVenc problem)
Bug: 27532497

Change-Id: I31e576ef9dc542df73aa6b0ea113d72724b50fc6
