Various fixes in {dh,dsa}_priv_encode()

Various fixes in {dh,dsa}_priv_encode()

Avoid creating an ASN1_STRING with negative length, set type, data
and length via ASN1_STRING_type_new() and ASN1_STRING_set0() instead
of doing this manually. Check return value for i2d_ASN1_INTEGER()
and use an intermediate ASN1_OBJECT instead of nested function calls.
Finally, clear sensitive data with freezero().

ok jsing