Guard against selection requestor disappearing

Guard against selection requestor disappearing

We made a number of unchecked accesses to the selection requestor,
which could cause X errors if the selection requestor vanished
(or we were sent invalid events). Add error traps around all of them.

See https://bugzilla.redhat.com/show_bug.cgi?id=1210814 for an
example of a crash that could be caused by this. (There are about
1000 instances of such a crash in http://retrace.fedoraproject.org
though no linked bug reports specifically talk about cut-and-paste
or DND.)

https://bugzilla.gnome.org/show_bug.cgi?id=756881