llvmpipe: Check allocated memory instead of argument

This is likely a copy-paste error.

CID: 1616035 Dereference after null check
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35091>

src/gallium/drivers/llvmpipe/lp_fence.c

@@ -291,7 +291,7 @@ lp_create_fence_fd(struct pipe_context *pipe,
 
    struct lp_fence *f = CALLOC_STRUCT(lp_fence);
 
-   if (!fence)
+   if (!f)
       goto fail;
 
    pipe_reference_init(&f->reference, 1);