Quick fix to correctly handle empty file/channel handles.

Empty file/channel handles (value < 0) should result in a negative
FileReference/ChannelReference value. The original code incorrectly
interpreted the negative reference value as an error, causing errno
to be returned. In most cases errno is 0, leading to confusion
across the wire when the deserialization logic attempts to lookup an
invalid, but non-empty reference.

Bug: None
Test: bufferhub_tests passes
Change-Id: I664daf3cdc178fe2c4ca0c44eef35a8bb9c7310d