FCoIB / Infiniband SDP: prevent a crash when 'enable manual settings' is set without filling address field

Bug: 10796
Change-Id: Ifdb7804ec5e1146b08f00e54647d802a69d52113
Reviewed-on: https://code.wireshark.org/review/5914
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>