video: hgafb: fix potential NULL pointer dereference

When ioremap fails, hga_vram should not be dereferenced. The fix
check the failure to avoid NULL pointer dereference.

Signed-off-by: Kangjie Lu <[e-mail]>
Cc: Aditya Pakki <[e-mail]>
Cc: Ferenc Bakonyi <[e-mail]>
[b.zolnierkie: minor patch summary fixup]
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>

drivers/video/fbdev/hgafb.c

···
 	hga_vram_len  = 0x08000;
 
 	hga_vram = ioremap(0xb0000, hga_vram_len);
+	if (!hga_vram)
+		goto error;
 
 	if (request_region(0x3b0, 12, "hgafb"))
 		release_io_ports = 1;