UDF: Fix a null pointer dereference in udf_sb_free_partitions

This patch fixes a regression caused by commit bff943af6fe "udf: Fix memory
leak when mounting" due to which it was triggering a kernel null point
dereference in case of interrupted mount OR when allocating memory to
sbi->s_partmaps failed in function udf_sb_alloc_partition_maps.

Reported-and-tested-by: James Hogan <james@albanarts.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -307,7 +307,8 @@ static void udf_sb_free_partitions(struct super_block *sb)
 {
 	struct udf_sb_info *sbi = UDF_SB(sb);
 	int i;
-
+	if (sbi->s_partmaps == NULL)
+		return;
 	for (i = 0; i < sbi->s_partitions; i++)
 		udf_free_partition(&sbi->s_partmaps[i]);
 	kfree(sbi->s_partmaps);