fix possible bad kref_put in amdgpu_uvd_ring_end_use

[Matthew Macy]

Clang identified this when I was merging up 4.8-rc1/rc2. I usually just disable warnings as they pop up as I treat the drivers as vendor code and FreeBSD's default clang settings are a bit on the anal side. However, this appears to be a legitimate bug. I pointed the problem out on #dri-devel and was asked to send a patch here. 

I haven't submitted patches before, but this is a trivial fix so bear with me.

>From 89ea7621c52ff9d3b6e48fa315609a042f2f5e0d Mon Sep 17 00:00:00 2001
From: Matt Macy <mmacy at nextbsd.org>
Date: Mon, 15 Aug 2016 23:22:49 -0700
Subject: [PATCH] drm/amdgpu: fix possible kref_put on random stack value

If amdgpu_uvd_get_create_msg fails fence_put(fence) will be called with
fence uninitialized - possibly leading to kref_put being called on whatever
value happens to be on the stack. Initializing fence to NULL precludes this.

Signed-off-by: Matt Macy mmacy at nextbsd.org
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
index b11f4e8..59931d4 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
@@ -1161,7 +1161,7 @@ void amdgpu_uvd_ring_end_use(struct amdgpu_ring *ring)
  */
 int amdgpu_uvd_ring_test_ib(struct amdgpu_ring *ring, long timeout)
 {
-	struct fence *fence;
+	struct fence *fence = NULL;
 	long r;
 
 	r = amdgpu_uvd_get_create_msg(ring, 1, NULL);
-- 
2.8.1