[C#] FasterLog Recovery from non-existent commit num fails assertion

[tli2]

When commitNum corresponds to an invalid (non-existent, deleted, or overwritten) commitNum, instead of throwing an exception or going down the supposed failure path (https://github.com/microsoft/FASTER/blob/master/cs/src/core/Index/CheckpointManagement/DeviceLogCommitCheckpointManager.cs#L161), recovery proceeds as if commitNum is valid and triggers an Assertion failure later (https://github.com/microsoft/FASTER/blob/master/cs/src/core/Index/Recovery/Recovery.cs#L929)