Fix memory leaks in DH code

Fix memory leaks in DH code

When used with OpenSSL v3.0.0+, the `openssldh_compare()`,
`openssldh_paramcompare()`, and `openssldh_todns()` functions
fail to cleanup the used memory on some error paths.

Use `DST_RET` instead of `return`, when there is memory to be
released before returning from the functions.