Fix phantom NRE from ProcessInput during some crashes on Mono

Pretty sure this only affected Mono. The infinite loop in ProgramRunLoop
continued even after the form was disposed (in the testing I did, this was
always during the call to CheckMessages). However, DisplayManager (actually
GlobalWin.DisplayManager) was cleared during disposal, so the call to
DisplayManager.UntransformPoint rightly threw an NRE. After this change, the
"real" stacktrace is still printed as always.

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -712,7 +712,7 @@ namespace BizHawk.Client.EmuHawk
 					Close();
 				}
 
-				if (_windowClosedAndSafeToExitProcess)
+				if (IsDisposed || _windowClosedAndSafeToExitProcess)
 				{
 					break;
 				}