Handling null pointer in gk20a_fence_is_expired. Bug 200117724 Change-Id: I0f9307a5f8b82bf990b6ddaea1a408d4f3f376fb Signed-off-by: Gagan Grover <ggrover@nvidia.com> Reviewed-on: http://git-master/r/777796 (cherry picked from commit dbf5bae53e0e7862754faba78eab84284786ecb3) Reviewed-on: http://git-master/r/795356 Reviewed-by: Automatic_Commit_Validation_User GVS: Gerrit_Virtual_Submit Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>