MAC/do: parse_rule_element(): Fix a panic, harden, simplify

The panic is caused by dereferencing 'element' at a point where it can
be NULL (if string ends at the ':').

Harden and simplify by enforcing the control flow rule in this function
that jumping to the end is reserved for error cases.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47605