util/iterator: allow retrieve last element

[XVilka]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

It is a bit clunky thing to do, but it is needed for Cutter, and easier to implement on the Rizin side.

See rizinorg/cutter#3298 for details

Test plan

CI is green

[imbillow]

I'm wondering if it's possible to free elem in rz_iterator_next.

[XVilka]

Good point, I removed the function call and used ->next() method instead.

[imbillow]

Suggested change

	elem = tail;
	it->free(elem);
	elem = tail;