fs_mgr: avb check avb_slot_data

Damaged avb metadata can result in avb_slot_verify returning a
nullptr in avb_slot_data.  Instead of an illegal access
violation in first_stage_init, we return the verify_result so
that it can be acted upon.

Test: confirm happenstance damaged vbmeta does not crash init.
Change-Id: I15be5bd32760bcc3418c5d8a943b016c0ddd56bc