Fix invalid indexing in object orientation editor

This could happen if the user enabled "Point to" and selected "Object"
without having an object selected. This can only happen when there is
only one object in the mission so it's only a minor bug but still
something worth fixing.

Author: asarium

fred2/orienteditor.cpp

@@ -215,9 +215,10 @@ void orient_editor::update_object(object *ptr)
 		loc.xyz.y = convert(m_location_y);
 		loc.xyz.z = convert(m_location_z);
 		if (((CButton *) GetDlgItem(IDC_POINT_TO_OBJECT))->GetCheck() == 1) {
-			v = Objects[index[m_object_index]].pos;
-			vm_vec_sub2(&v, &ptr->pos);
-
+			if (m_object_index >= 0) {
+				v = Objects[index[m_object_index]].pos;
+				vm_vec_sub2(&v, &ptr->pos);
+			}
 		} else if (((CButton *) GetDlgItem(IDC_POINT_TO_LOCATION))->GetCheck() == 1) {
 			vm_vec_sub(&v, &loc, &ptr->pos);