YAPF: Prevent user for configuring signal penalties to negative values

Warn if signal penalty polynomial goes negative, and clamp to non-negative

src/saveload/afterload.cpp

@@ -3795,6 +3795,9 @@ bool AfterLoadGame()
 	InvalidateVehicleTickCaches();
 	ClearVehicleTickCaches();
 
+	extern void YapfCheckRailSignalPenalties();
+	YapfCheckRailSignalPenalties();
+
 	/* Show this message last to avoid covering up an error message if we bail out part way */
 	switch (gcf_res) {
 		case GLC_COMPATIBLE: ShowErrorMessage(STR_NEWGRF_COMPATIBLE_LOAD_WARNING, INVALID_STRING_ID, WL_CRITICAL); break;