sys-utils: fix add NULL check for mnt_fs_get_target return value

sys-utils: fix add NULL check for mnt_fs_get_target return value

The static analyzer flagged a potential issue: the return value of
mnt_fs_get_target(fs) could be NULL, but it was dereferenced without
a check. This could lead to undefined behavior.

Added a NULL check before using the tgt pointer. If tgt is NULL,
the current iteration is skipped.

ChanChanges:
- Added if (!tgt) check before using tgt.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>