Fix an off-by-one error in vm_map_pmap_enter().

If the starting pindex is equal to object->size, there is nothing to do.
This was harmless since the rest of vm_map_pmap_enter() has no effect
when psize == 0.

Submitted by: Wuyang Chung <wuyang.chung1@gmail.com>
Reviewed by: alc, dougm, kib
MFC after: 1 week
Github PR: https://github.com/freebsd/freebsd/pull/417
Differential Revision: https://reviews.freebsd.org/D22678