[multipath] fix segfault upon '-ll' with empty map present
[multipath] fix segfault upon '-ll' with empty map present

Noted by Maxim Kozover, regression testing squad


vijaychauhan/multipath-tools,vijaychauhan/multipath-tools,unakatsuo/multipath-tools,unakatsuo/multipath-tools,grzn/multipath-tools-explained,gebi/multipath-tools,grzn/multipath-tools-explained,unakatsuo/multipath-tools