clean up *sig_info handling in link_socket_init_phase2()

clean up *sig_info handling in link_socket_init_phase2()

The code was a mix of "assume that it is not NULL" and "check that
it is not NULL before using" - it cannot be NULL (due to the single
call graph, referencing c->sig with the global context), but for
good measure, add an ASSERT() upon function entry and get rid of
all the individual checks.

Found by Coverity.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1478555042-31299-1-git-send-email-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12931.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>