dnssd: Fix crashes when mount failed
dnssd: Fix crashes when mount failed

Mount operation might fail if you run multiple mount operations
concurrently. Backend memory is released consequently. Unfortunatelly
AvahiServiceResolvers are not freed and thus resolve_callback might
be called after backend finalize, which causes segmentation faults.
This patch fixes this and also some other memory leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=631023

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs