afsocket: fix segmentation fault in the notls build

When SSL is disabled, the construct_transport() virtual method
was incorrectly initialized, causing an endless recursion during
initialization of any AF_INET destination. This in turn causes a stack
exhaustion, which in turn causes a segmentation fault.

Thanks to Howard for the detailed bug report and proposed fix.

Reported-by: Howard Tsai <htsai@skytap.com>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>