Add null pointer check when creating SocketAddress

Summary:
If for some reason SocketAddress:make_address() fails to create a proper
sockaddr_un structure then SocketAddress::address() will return a null
pointer. In such case, to prevent application crash, check for null pointer
prior to any socket operation.

Reviewers: dfaure, ossi

Reviewed By: dfaure, ossi

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12744