Dereference of null pointer in network/nethost.cc

[Hans Joachim Desserud]

Found by scan-build (llvm 3.3) in r6594.

[Nasenbaer (nasenbaer)]

Definitively a false positive:
it assumes g_fs->IsDirectory(mapfilename) is true (first) as well as false (second), although mapfilename was not changed inbetween. Maybe scan-build thinks Widelands is multithreaded where other branches might change one of the variables?