doom: Fix behavior of M_EPI4 menu item.

Ultimate Doom's doom.exe will crash when run with an IWAD that doesn't
contain M_EPI4 (like the stock v1.9 IWAD).
Thanks to Alexandre Xavier for the bug report (fixes #344).

src/doom/m_menu.c

@@ -2113,8 +2113,6 @@ void M_Init (void)
 	// Episode 2 and 3 are handled,
 	//  branching to an ad screen.
       case registered:
-	// We need to remove the fourth episode.
-	EpiDef.numitems--;
 	break;
       case retail:
 	// We are fine.
@@ -2122,6 +2120,15 @@ void M_Init (void)
 	break;
     }
 
+    // Versions of doom.exe before the Ultimate Doom release only had
+    // three episodes; if we're emulating one of those then don't try
+    // to show episode four. If we are, then do show episode four
+    // (should crash if missing).
+    if (gameversion < exe_ultimate)
+    {
+	EpiDef.numitems--;
+    }
+
     opldev = M_CheckParm("-opldev") > 0;
 }