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