ephy-title-box: Fix crash when closing second window

These binding could already have been destroyed. It might be safe to
simply remove the calls to g_clear_object, since they are not needed for
memory management, but then the bindings might not be unbound if a new
web view is set before the old one is destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=732784
ephy-title-box: Fix a warning when closing window

From some reason it look like the disconnect by sig_id
plot a warning here, since we can to disconnect by func
it better here.

https://bugzilla.gnome.org/show_bug.cgi?id=732784