EDAC, ghes: Add a null pointer check in ghes_edac_unregister()

EDAC, ghes: Add a null pointer check in ghes_edac_unregister()

Add a null check for ghes_pvt, before dereferencing it. The pointer
could still be null in case the return path is taken before initialising
ghes_pvt in the registration function.

Reviewed-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: lkml <linux-kernel@vger.kernel.org>
Link: http://lkml.kernel.org/r/1524737809-24475-1-git-send-email-sughosh.ganu@arm.com
Signed-off-by: Borislav Petkov <bp@suse.de>