Fix invalid pointer use in get_my_hostname().

clang-3.7 warnings surfaced an actual bug:

invitation.c:185:5: error: address of array 'filename' will always evaluate to 'true'
      [-Werror,-Wpointer-bool-conversion]
        if(filename) {
        ~~ ^~~~~~~~

The regression was introduced in 3ccdf50beb6b2d3f2730bdc66006b43190537cde.