libsnapshot: Fix CHECK failure during second phase merge

This CHECK prevents a release build from resuming a two-phase merge if
the merge initially failed in the first pass.

Bug: 213031779
Bug: 213253413
Bug: 193549218
Ignore-AOSP-First: cherry-pick from AOSP
Test: vts_libsnapshot_test
Test: update_engine_unittests
Change-Id: I8bf00e3016546ef7039bb0b18eb977cc3dc1066a
Merged-In: I8bf00e3016546ef7039bb0b18eb977cc3dc1066a