libsnapshot: Skip initializing snapshot if not created.

If no opeartions is to be executed on a partition, it is
possible that no snapshot is created for it. In that case,
don't crash when initalizing snapshots.

Test: libsnapshot_test
Change-Id: I7bb0b5596aa6e4eca319e9c88f6552d1907a6944
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index 0200077..354aafa 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -1977,7 +1977,7 @@
         }
 
         auto it = all_snapshot_status.find(target_partition->name());
-        CHECK(it != all_snapshot_status.end()) << target_partition->name();
+        if (it == all_snapshot_status.end()) continue;
         cow_params.partition_name = target_partition->name();
         std::string cow_name;
         if (!MapCowDevices(lock, cow_params, it->second, &created_devices_for_cow, &cow_name)) {