file monitors: reorder some code to avoid segfault
file monitors: reorder some code to avoid segfault

We must initialise '->source' before we use fields inside of it.

https://bugzilla.gnome.org/show_bug.cgi?id=758823

johne53/MB3Glib,johne53/MB3Glib,mzabaluev/glib,johne53/MB3Glib,endlessm/glib,Distrotech/glib,Distrotech/glib,endlessm/glib,MathieuDuponchelle/glib,endlessm/glib,Distrotech/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,endlessm/glib,johne53/MB3Glib,Distrotech/glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,mzabaluev/glib,mzabaluev/glib,endlessm/glib,mzabaluev/glib,Distrotech/glib,johne53/MB3Glib,mzabaluev/glib