vm_page: Fix loading bad memory addresses from file

When loading bad memory addresses from a file, we are passed an end
pointer that points on the first byte after the buffer. We want the
buffer to be null-terminated (by changing the last byte to \0 if it is
reasonable to do so), so adjust the end pointer to be on that byte.

Approved by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51433