vm_page_grab_valid(): clear *mp in case of pager denying page allocation

Same as it is done in other error return cases. Callers depend on error
case returning NULL, e.g. vm_imgact_hold_page().

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37719