dma-buf: Fix NULL pointer dereference in sanitycheck()

If due to a memory allocation failure mock_chain() returns NULL, it is
passed to dma_fence_enable_sw_signaling() resulting in NULL pointer
dereference there.

Call dma_fence_enable_sw_signaling() only if mock_chain() succeeds.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: d62c43a9 ("dma-buf: Enable signaling on fence for selftests")
Signed-off-by: Pavel Sakharov <p.sakharov@ispras.ru>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240319231527.1821372-1-p.sakharov@ispras.ru

drivers/dma-buf/st-dma-fence-chain.c

@@ -84,11 +84,11 @@ static int sanitycheck(void *arg)
 		return -ENOMEM;
 
 	chain = mock_chain(NULL, f, 1);
-	if (!chain)
+	if (chain)
+		dma_fence_enable_sw_signaling(chain);
+	else
 		err = -ENOMEM;
 
-	dma_fence_enable_sw_signaling(chain);
-
 	dma_fence_signal(f);
 	dma_fence_put(f);