Fix savestating being possible after movie has finished

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

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 
@@ -91,13 +92,7 @@ public void Create(string filename, SaveStateConfig config)
 				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}!");
-						}
+						Debug.Assert(_movieSession.Movie.FrameCount >= _emulator.Frame);
 						// this never should have been a core's responsibility
 						tw.WriteLine("Frame {0}", _emulator.Frame);
 						_movieSession.HandleSaveState(tw);

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -4331,6 +4331,12 @@ public void SaveState(string path, string userFriendlyStateName, bool fromLua =
 				return;
 			}
 
+			if (MovieSession.Movie.IsFinished())
+			{
+				OSD.AddMessage("Cannot savestate after movie end!");
+				return;
+			}
+
 			try
 			{
 				new SavestateFile(Emulator, MovieSession, MovieSession.UserBag).Create(path, Config.Savestates);