There are times when a len==0 parameter to mmap is okay. But on a
32-bit machine, a len parameter just a few bytes short of 4G, rounded
up to a page boundary and hitting zero then, is not okay. Return
failure in that case.

Reported by: pho
Reviewed by: alc, kib (mentor)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D20580