zram: check comp is non-NULL before calling comp_destroy

This is a pre-requisite for the backport of commit 74363ec674cb ("zram:
fix uninitialized ZRAM not releasing backing device"), which has been
implemented differently in commit 7ac07a26dea7 ("zram: preparation for
multi-zcomp support") upstream.

We only need to ensure that zcomp_destroy is not called with a NULL
comp, so add this check as the other commit cannot be backported easily.

Stable-dep-of: 74363ec674cb ("zram: fix uninitialized ZRAM not releasing backing device")
Link: https://lore.kernel.org/[email protected]
Suggested-by: Kairui Song <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: martinetd

drivers/block/zram/zram_drv.c

@@ -1733,7 +1733,8 @@ static void zram_reset_device(struct zram *zram)
 	zram_meta_free(zram, zram->disksize);
 	zram->disksize = 0;
 	memset(&zram->stats, 0, sizeof(zram->stats));
-	zcomp_destroy(zram->comp);
+	if (zram->comp)
+		zcomp_destroy(zram->comp);
 	zram->comp = NULL;
 	reset_bdev(zram);