jbd2: use NULL instead of 0 when pointer is needed

Fixes sparse warning:

fs/jbd2/journal.c:1892:9: warning: Using plain integer as NULL pointer
Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: linux-ext4@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

fs/jbd2/journal.c

@@ -1889,7 +1889,7 @@ static struct kmem_cache *get_slab(size_t size)
 	BUG_ON(i >= JBD2_MAX_SLABS);
 	if (unlikely(i < 0))
 		i = 0;
-	BUG_ON(jbd2_slab[i] == 0);
+	BUG_ON(jbd2_slab[i] == NULL);
 	return jbd2_slab[i];
 }