Avoid error if a 64K transfer is done from a page-unaligned buffer.
Avoid error if a 64K transfer is done from a page-unaligned buffer.

Encountered with savecore and fix tested by bluhm@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars