gnetworkmonitornetlink: Fix a memory leak in unusual circumstances

If the GNetworkMonitorNetlink is finalised part-way through a dump
(after request_dump() is called, but before finish_dump() is called),
dump_networks was leaked. Fix that.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793880