Fix double free in cert query

[ArnoStiefvater]

What:

Double free in cert_query.
ksba_cert_get_image does not return a copy but only a pointer to the image. Do not free it with gnutls_free. It will get freed with ksba_cert_release when nasl_cert_close is called.

Why:

Fix #480.

How:

Test with some VT

if(description) {
  script_oid("1.2.3");
  exit(0);
}

cert = "<Some BER encoded certificate>";
if( ! cert = cert_open( cert) )
  display( "Error opening cert!" );

size = cert_query( cert, "key-size" ); 
display(size);

cert_close( cert );
exit(0);

Error double free or corruption (!prev) will be gone after applied PR.

Checklist:

• Tests
• CHANGELOG Entry