rsa1: rsa_loadpriv_core: Always free b, and zero things we free

rsa1: rsa_loadpriv_core: Always free b, and zero things we free

Make the FREE macro zero its argument.  This makes it idempotent.

This means that at the end of the function, and at all points where
LD* is called, b is either from malloc and freeable, or null.  It is
never a free'd pointer.

So we can free it.  This is moving us towards always unconditionally
freeing everything on the exit path, to support non-fatal early
return.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>