Handle zero snapshot size appropriately.

This won't happen in practice because device size is never
zero. Hypothetically, if it is, PartitionCowCreator still
suggests that a CoW size of 8192 bytes needs to be created. In
reality, it is not necessary.

Test: libsnapshot_fuzzer
Bug: 155484992
Change-Id: I6c69f54820522d50c699384eec90c474ca3a9402