Fix heap OOB read in macho.iterate_chained_fixups ##crash

* Reported by peacock-doris via huntr.dev
* Reproducer 'tests_65305'

mrmacete:
* Return early if segs_count is 0
* Initialize segs_count also for reconstructed fixups

Co-authored-by: pancake <pancake@nopcode.org>
Co-authored-by: Francesco Tamagni <mrmacete@protonmail.ch>