Avoid exception in key handling routine

src/MacVim/MMTextViewHelper.m

@@ -747,7 +747,7 @@ - (void)dispatchKeyEvent:(NSEvent *)event
     NSString *chars = [event characters];
     NSString *unmodchars = [event charactersIgnoringModifiers];
     unichar c = [chars characterAtIndex:0];
-    unichar imc = [unmodchars characterAtIndex:0];
+    unichar imc = [unmodchars length] > 0 ? [unmodchars characterAtIndex:0] : 0;
     int len = 0;
     const char *bytes = 0;
     int mods = [event modifierFlags];