libbe: fix be_mounted_at() with props after bootonce

propinfo.bootonce was filled with garbage, leading to a segfault later
during prop building. Initialize it to NULL.

Approved by: re (gjb)

(cherry picked from commit d155d8e138563c0b39c658684e6a4fe6e2e36f8a)
(cherry picked from commit 6ce8e35cead46048d9ae3c1dabafe1ef4ed26f97)