io: Fix check if library is running inside build tree
io: Fix check if library is running inside build tree

The existing check was missing the length of the string so instead of reading
the end of the string it read the memory before it.

https://bugzilla.gnome.org/show_bug.cgi?id=733413

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf