Don't show the FPS counter during startup.

This fixes a sigsegv on startup when the HUD is set to be transparent. Thanks to toaster for helping me track down the bug!

src/screen.c

@@ -526,6 +526,9 @@ void SCR_DisplayTicRate(void)
 	INT32 ticcntcolor = 0;
 	const INT32 h = vid.height-(8*vid.dupy);
 
+	if (gamestate == GS_NULL)
+		return;
+
 	for (i = lasttic + 1; i < TICRATE+lasttic && i < ontic; ++i)
 		fpsgraph[i % TICRATE] = false;