This rewrites two "for" loops in BN_rshift() - equality with zero is

generally a more efficient comparison than comparing two integers, and the
first of these two loops was off-by-one (copying one too many values). This
change also removes a superfluous assignment that would set an unused word
to zero (and potentially allow an overrun in some cases).

Submitted by: Nils Larsch
Reviewed by: Geoff Thorpe