The attached patch fixes an overflow bug in DecodeInterval when applying
the units week, decade, century, and millennium. The overflow check logic
was modelled after the overflow check at the beginning of `int
tm2interval(struct pg_tm *tm, fsec_t fsec, Interval *span);` in timestamp.c.
This is my first patch, so apologies if I screwed up the process somewhere.
- Joe Koshakow