Handle trashed queue pointers in vm_page_acquire_unlocked().

vm_page_acquire_unlocked() relies on type-stability of vm_page
structures and assumes that the listq linkage pointers always point to a
vm_page or are NULL. QUEUE_MACRO_DEBUG_TRASH breaks that assumption, so
add an explicit check for a trashed queue pointer before dereferencing.

Reported and tested by: pho
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24472