UDF: Fix a null pointer dereference in udf_sb_free_partitions

This patch fixes a regression caused by commit bff943af

 "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>