Fix potential crash in savestate loading

Also throw when attempting to create such an invalid savestate. No idea yet how that can happen but apparently it can.

src/BizHawk.Client.Common/movie/bk2/Bk2Movie.InputLog.cs

@@ -135,6 +135,12 @@ namespace BizHawk.Client.Common
 				stateFrame = newLog.Count;  // In case the frame count failed to parse, revert to using the entire state input log
 			}
 
+			if (stateFrame > newLog.Count)
+			{
+				errorMessage = $"Savestate has invalid frame number {stateFrame} (expected maximum {newLog.Count})";
+				return false;
+			}
+
 			if (Log.Count < stateFrame)
 			{
 				if (this.IsFinished())

src/BizHawk.Client.Common/savestates/SavestateFile.cs

@@ -104,6 +104,13 @@ namespace BizHawk.Client.Common
 				bs.PutLump(BinaryStateLump.Input,
 					tw =>
 					{
+						// TODO: this should not happen and no exception should be thrown here.
+						// Just make this noisy for now until the issue is fixed.
+						if (_movieSession.Movie.FrameCount < _emulator.Frame)
+						{
+							throw new InvalidOperationException(
+								$"Tried to create a savestate at frame {_emulator.Frame}, but only got a log of length {_movieSession.Movie.FrameCount}!");
+						}
 						// this never should have been a core's responsibility
 						tw.WriteLine("Frame {0}", _emulator.Frame);
 						_movieSession.HandleSaveState(tw);