drm/savage: dereferencing an error pointer

A recent cleanup changed the kmalloc() + copy_from_user() to
memdup_user() but the error handling wasn't updated so we might call
kfree(-EFAULT) and crash.

Fixes: a6e3918bcdb1 ('GPU-DRM-Savage: Use memdup_user() rather than duplicating')
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/20161012062227.GU12841@mwanda

drivers/gpu/drm/savage/savage_state.c

···
 		kvb_addr = memdup_user(cmdbuf->vb_addr, cmdbuf->vb_size);
 		if (IS_ERR(kvb_addr)) {
 			ret = PTR_ERR(kvb_addr);
+			kvb_addr = NULL;
 			goto done;
 		}
 		cmdbuf->vb_addr = kvb_addr;