nouveau: Fix crash when pci_ram_rom() returns a size of 0

From b15b244d6e6e20964bd4b85306722cb60c3c0809 Mon Sep 17 00:00:00 2001
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Mon, 2 Apr 2012 13:28:18 +1000
Subject:

Under some circumstances, pci_map_rom() can return a valid mapping
but a size of 0 (if it cannot find an image in the header).

This causes nouveau to try to kmalloc() a 0 sized pointer and
dereference it, which crashes.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bios.c

@@ -177,14 +177,15 @@ bios_shadow_pci(struct nvbios *bios)
 
 	if (!pci_enable_rom(pdev)) {
 		void __iomem *rom = pci_map_rom(pdev, &length);
-		if (rom) {
+		if (rom && length) {
 			bios->data = kmalloc(length, GFP_KERNEL);
 			if (bios->data) {
 				memcpy_fromio(bios->data, rom, length);
 				bios->length = length;
 			}
-			pci_unmap_rom(pdev, rom);
 		}
+		if (rom)
+			pci_unmap_rom(pdev, rom);
 
 		pci_disable_rom(pdev);
 	}