libsnapshot: Validate Sequence Op data

If the sequence op contains a Cow Operation that doesn't exist, we
should log the error and abort. Otherwise we'll try to merge an op that
doesn't exist.

Test: cow_api_test CowTest.MissingSeqOp
Bug: 177104308
Change-Id: I1a96f09798bc16126ae1338c6259506b18c88f51
2 files changed