Allow the game to continue even if the `STARTUP` lump is somehow missing

src/d_main.c

@@ -731,6 +731,7 @@ tic_t rendergametic;
 void D_SRB2Loop(void)
 {
 	tic_t oldentertics = 0, entertic = 0, realtics = 0, rendertimeout = INFTICS;
+	static lumpnum_t gstartuplumpnum;
 
 	if (dedicated)
 		server = true;
@@ -770,7 +771,12 @@ void D_SRB2Loop(void)
 	*/
 	/* Smells like a hack... Don't fade Sonic's ass into the title screen. */
 	if (gamestate != GS_TITLESCREEN)
-		V_DrawScaledPatch(0, 0, 0, W_CachePatchNum(W_GetNumForName("STARTUP"), PU_PATCH));
+	{
+		gstartuplumpnum = W_CheckNumForName("STARTUP");
+		if (gstartuplumpnum == LUMPERROR)
+			gstartuplumpnum = W_GetNumForName("MISSING");
+		V_DrawScaledPatch(0, 0, 0, W_CachePatchNum(gstartuplumpnum, PU_PATCH));
+	}
 
 	for (;;)
 	{