fix "can't paste from clipboard with only one row in clipboard"

fix "can't Undo to a blank NoteField"
clamp paste area so that pasting past the maximum beat is impossible

stepmania/src/ScreenEdit.h

@@ -179,6 +179,7 @@ protected:
 	int g_iShiftAnchor;
 
 	NoteData			m_Clipboard;
+	bool				m_bHasUndo;
 	NoteData			m_Undo;
 
 	RageSound			m_soundAddNote;