Move the malloc and free of scratch space out of the internal_mul

Move the malloc and free of scratch space out of the internal_mul
routines into their callers, where they'll be done once for a whole
modpow rather than many times within each multiply. Doesn't save much
time as far as I can see - perhaps a couple of percent, one second in
the minute it takes to run the new bignum test suite - but seems like
a sensible idea anyway on general principles.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9103 cda61777-01e9-0310-a592-d414129be87e