libsnapshot: Fix merge failure when interrupted before the first-phase switchover.

If the merge is interrupted in between committing the merge state, and
converting first-phase snapshots to merge targets, then
CheckTargetMergeState() gets very confused. It assumes all snapshots
must be part of the second merge phase, and errors out.

Bug: 352539864
Test: vts_libsnapshot_test
Change-Id: Ib406868e559d56e3179281eb28668d9c0b9f7947