STIR/SHAKEN: OPENSSL_free serial hex from openssl.

We're getting the serial number of the certificate from openssl and
freeing it with ast_free(), but it needs to be freed with OPENSSL_free()
instead. Now we duplicate the string and free the one from openssl with
OPENSSL_free(), which means we can still use ast_free() on the returned
string.

https://wiki.asterisk.org/wiki/display/AST/OpenSIPit+2021

Change-Id: Ia6e1a4028c1933a0e1d204b769ebb9f5a11f00ab