DSA_dup_DH() duplicates DSA parameters/keys as DH parameters/keys. q
is lost during that conversion, but the resulting DH parameters
contain its length.
RETURN VALUE
DSA_dup_DH() returns the new DH structure, and NULL on error. The
error codes can be obtained by ERR_get_error(3).
NOTE
Be careful to avoid small subgroup attacks when using this.
