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.comSigned-off-by: Borislav Petkov <bp@suse.de>

drivers/edac/ghes_edac.c

@@ -517,6 +517,9 @@ void ghes_edac_unregister(struct ghes *ghes)
 {
 	struct mem_ctl_info *mci;
 
+	if (!ghes_pvt)
+		return;
+
 	mci = ghes_pvt->mci;
 	edac_mc_del_mc(mci->pdev);
 	edac_mc_free(mci);