Add error-checking to timestamp_recv

Greetings,

  The attached patch adds some error-checking to the timestamp_recv
  function so that it's not possible to put an invalid timestamp into a
  timestamp column (hopefully).  The check is done in basically the
  exact same way the out-of-bounds check in timestamp2tm is done.
  There's probably an easier/cleaner way but this should work or at
  least generate discussion and a better patch. :)

      Thanks,

        Stephen