Checked return value of XGetWindowProperty and return false if it fails.

Return value of "XGetWindowProperty(im->core.display, spec->lib_connect_wid, prop, 0L, (length + bytes_after_ret + 3UL) / 4UL, 1, 0UL, &type_ret, &format_ret, &nitems, &bytes_after_ret, &prop_ret)" is not checked
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Erkki Seppälä <erkki.seppala@vincit.fi>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>