Prevent an out of range exception from being thrown. (was commonly being thrown when commit was used on the last row of the grid)

Originally committed to SVN as r2945.

aegisub/src/base_grid.cpp

@@ -224,6 +224,7 @@ void BaseGrid::ClearSelection() {
 /////////////////////////
 // Is cell in selection?
 bool BaseGrid::IsInSelection(int row, int col) const {
+	if (row >= GetRows() || row < 0) return false;
 	(void) col;
 	try {
 		return selMap.at(row);