agenda: Handle wrong-type-arg in select.

agenda: Handle wrong-type-arg in select.

This fixes

Backtrace:
          10 (apply-smob/1 #<catch-closure 7fca5dcad920>)
In ice-9/boot-9.scm:
    705:2  9 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  8 (_ #(#(#<directory (guile-user) 7fca5d93f140>)))
In ice-9/boot-9.scm:
   2312:4  7 (save-module-excursion _)
  3832:12  6 (_)
In 8sync/actors.scm:
    812:6  5 (run-hive #<<hive> 7fca5d977190> _ #:cleanup _ #:handle-signals _)
In ice-9/control.scm:
    91:24  4 (call-with-escape-continuation _)
In 8sync/agenda.scm:
    569:6  3 (run-agenda #<<agenda> queue: (() . #f) prompt-tag: ("prompt") read-port-map: #<hash-table 7fca5…> …)
    470:7  2 (update-agenda-from-select! #<<agenda> queue: (() . #f) prompt-tag: ("prompt") read-port-map: #<h…>)
In ice-9/boot-9.scm:
    829:9  1 (catch system-error #<procedure 7fca5bf1ab70 at 8sync/agenda.scm:471:9 ()> #<procedure 7fca5cd09…> …)
In unknown file:
           0 (select (#<closed: file 7fca5b077380> #<input-output: socket 11>) (#<closed: file 7fca5b077380>) # …)

ERROR: In procedure select:
In procedure select: Wrong type argument in position 1: #<closed: file 7fca5b077380>

* 8sync/agenda.scm (update-agenda-from-select!): Catch everything.