fix: #972 next-slide-id fails when max used

https://github.com/scanny/python-pptx/issues/972

Naive "max + 1" slide-id allocation algorithm assumed that slide-ids
were assigned from bottom up. Apparently some client assigns slide ids
from the top (2,147,483,647) down and the naive algorithm would assign
an invalid slide-id in that case.

Detect when the assigned id is out-of-range and fall-back to a robust
algorithm for assigning a valid id based on a "first unused starting at
bottom" policy.