Fix file transfer corner cases.

Fix zero packet read/write in corner cases
with initial read/writes. This affects transfers
of 16372 bytes to the device and transfers of
1012 bytes (or packet size - 12) from the device.

Test: Transfer various length files
Bug: 37175601
Change-Id: Ic1281e8757d1a33d78c66d2c57319b5ee38d6a46