video: fbdev: sis: fix potential NULL dereference in sisfb_post_sis300()

Do no access bios[] if it's NULL.
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/sis/sis_main.c

@@ -4463,7 +4463,7 @@ static void sisfb_post_sis300(struct pci_dev *pdev)
 		SiS_SetReg(SISCR, 0x37, 0x02);
 		SiS_SetReg(SISPART2, 0x00, 0x1c);
 		v4 = 0x00; v5 = 0x00; v6 = 0x10;
-		if(ivideo->SiS_Pr.UseROM) {
+		if (ivideo->SiS_Pr.UseROM && bios) {
 			v4 = bios[0xf5];
 			v5 = bios[0xf6];
 			v6 = bios[0xf7];