Add and use and unlikely-to-be-eliminated memwipe()

Apparently some compilers like to eliminate memset() operations on
data that's about to go out-of-scope.  I've gone with the safest
possible replacement, which might be a bit slow.  I don't think this
is critical path in any way that will affect performance, but if it
is, we can work on that in 0.2.4.

Fixes bug 7352.