security: fix crashes during DH generation

When DH parameters are generated an error can occures
and an empty (default constructed) DH params instance
is returned.
This causes a crashes into ring account code that not
handles such case.

This patch fixes this issue by adding operator bool()
method to DH params and checking if not false when
trying to save them.

Note: If no DH params are generated, the empty instance
is given up to the TlsSession object.
Check if your cipher suite could handle empty DH params!

Change-Id: I98c9c0317f4b8cb107ea0bca5f94b69321cbba76
Tuleap: #792