Fix zero length packet handling

Ensure that a zero length packet is properly sent
/ received when handling data that is a multiple of the
endpoint size.

This fixes an issue where files whose size is a multiple
of the endpoint size would hang during transfer.

Bug: 37175601
Test: check successful file transfer.
Change-Id: Iaf3cea18422234d65b59f5961ef9fb3f3ee2049a