crt_malloc: more accurate handling of mmap(2) failure

Reset both pagepool_start and pagepool_end after a mmap(2) failure,
to avoid using invalid pagepool either for allocation or munmap(2).

PR: 259076
Noted by: Denis Koreshkov <dynamic-wind@mail.ru>
Reviewed by: arichardson
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32514