tee: shm: don't put_page on null shm->pages

In the case that shm->pages fails to allocate, the current exit
error path will try to put_page on a null shm->pages and cause
a null pointer dereference when accessing shm->pages[n]. Fix this
by only performing the put_page and kfree on shm->pages if it
is not null.

Detected by CoverityScan, CID#1463283 ("Dereference after null check")

Fixes: 033ddf12bcf5 ("tee: add register user memory")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

drivers/tee/tee_shm.c

@@ -335,9 +335,11 @@ err:
 			idr_remove(&teedev->idr, shm->id);
 			mutex_unlock(&teedev->mutex);
 		}
-		for (n = 0; n < shm->num_pages; n++)
-			put_page(shm->pages[n]);
-		kfree(shm->pages);
+		if (shm->pages) {
+			for (n = 0; n < shm->num_pages; n++)
+				put_page(shm->pages[n]);
+			kfree(shm->pages);
+		}
 	}
 	kfree(shm);
 	teedev_ctx_put(ctx);