Catch overflow when rounding intervals in AdjustIntervalForTypmod.

Catch overflow when rounding intervals in AdjustIntervalForTypmod.

Previously, an interval microseconds field close to INT64_MAX or
INT64_MIN could overflow, producing a result with not even the
correct sign, while being rounded to match a precision specification.

This seems worth fixing, but not worth back-patching, in part
because the ereturn() notation doesn't exist very far back.

Report and patch by Joseph Koshakow (some cosmetic mods by me)

Discussion: https://postgr.es/m/CAAvxfHfpuLgqJYzkUcher466Z1LpmE+5Sm+zc8L6zKCOQ+6TDQ@mail.gmail.com