libstdc++: Avoid overflow in ranges::advance(i, n, bound)

libstdc++: Avoid overflow in ranges::advance(i, n, bound)

When (bound - i) or n is the most negative value of its type, the
negative of the value will overflow. Instead of abs(n) >= abs(bound - i)
use n >= (bound - i) when positive and n <= (bound - i) when negative.
The function has a precondition that they must have the same sign, so
this works correctly. The precondition check can be moved into the else
branch, and simplified.

The standard requires calling ranges::advance(i, bound) even if i==bound
is already true, which is technically observable, but that's pointless.
We can just return n in that case. Similarly, for i!=bound but n==0 we
are supposed to call ranges::advance(i, n), but that's pointless. An LWG
issue to allow omitting the pointless calls is expected to be filed.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (ranges::advance): Avoid signed
	overflow. Do nothing if already equal to desired result.
	* testsuite/24_iterators/range_operations/advance_overflow.cc:
	New test.