This happens when dropping files from dtfile, which somehow puts
non-atom stuff in window properties of type ATOM_PAIR.
* src/xselect.c (x_atom_to_symbol): Catch errors around
XGetAtomName.
return QNULL;
block_input ();
+ x_catch_errors (dpyinfo->display);
str = XGetAtomName (dpyinfo->display, atom);
+ x_uncatch_errors ();
unblock_input ();
TRACE1 ("XGetAtomName --> %s", str);
if (! str) return Qnil;