libsnapshot: Merge completion for sector 0

Snapuserd daemon parses the merge completion request based on
how the dm-snapshot merge is done. dm-snapshot marks the merge as
complete by zeroing out the metadata viz old-chunk and new-chunk id's.

If we have a sector 0 operation such as copy/replace op,
then old-chunk id will be 0 and new-chunk id will be a non-zero
pseudo number. Once the merge is complete, then old-chunk and new-chunk will be 0.

The problem is that daemon used to track the merge completion just by checking
if old-chunk was non-zero. This check is not sufficient and ends up
tripping the assert in the daemon.

Bug: 178061207
Test: Modify cow_snapuserd_test to test this case and validate the
      IO path.
Reported-by: Kelvin Zhang <zhangkelvin@google.com>
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I6603af1c7b55e487dc3aec0c30c0a9dea0fedb56