Fix overflow hazard in timestamp_pl_interval

Hi all,

Attached is a patch that fixes some overflow/underflow hazards in
`timestamp_pl_interval`. The microseconds overflow could produce
incorrect result. The month overflow would generally still result in an
error from the timestamp month field being too low, but it's still
better to catch it early.

I couldn't find any existing timestamp plus interval tests so I stuck
a new tests in `timestamp.sql`. If there's a better place, then
please let me know.

Thanks,
Joe Koshakow