fs: ext4: Fix free(NULL)

The 'depth_dirname', 'ptr', 'parent_inode' and 'first_inode' pointers
may be null. Thus, it is necessary to check them before using free() to
avoid free(NULL) cases.

Fixes: 934b14f2 ("ext4: free allocations by parse_path()")
Signed-off-by: Mikhail Ilin <ilin.mikhail.ol@gmail.com>