libsnapshot: Zero out leftover ops in CowWriter

When opening in append mode, we could write less than what was present
before. This could result in data blocks referencing beyond the end of
the file, or partially written ops. Zeroing these out will prevent
invalid leftovers from potentially causing confusion.

Bug: 183985866
Test: cow_api_test
Change-Id: I56f0218f3ea5b83c0614d1b86e81a4ca885f5c5e