dh-gex: Fix memory leak in DH GEX with OpenSSL
When using OpenSSL, the bignums generated during group exchange are
duplicated and don't transfer the memory management responsibility to
the back-end. The original generated bignums can be freed.
The leak was detectable by running:
$ valgrind --leak-check=full ./tests/pkd/pkd_hello -i1 \
Signed-off-by: Anderson Toshiyuki Sasaki <email@example.com>
Reviewed-by: Andreas Schneider <firstname.lastname@example.org>