Don't let us warp to a map that doesn't exist (really!)

Okay so 6464df9876e472d1210aabce4237d02af38377e1, I WAS mistaken! Except that's
not how you check for a map's existence, at least not how the old map command
did it.

(cherry picked from commit 566b4a1626399b1c1621bf8b4ab1f8426c789c36)

src/g_game.c

@@ -4972,6 +4972,10 @@ INT32 G_FindMapByNameOrCode(const char *mapname, char **realmapnamep)
 
 	if (usemapcode)
 	{
+		/* we can't check mapheaderinfo for this hahahaha */
+		if (W_CheckNumForName(G_BuildMapName(newmapnum)) == LUMPERROR)
+			return 0;
+
 		if (realmapnamep)
 			(*realmapnamep) = G_BuildMapTitle(newmapnum);
 	}