Fix issue 16942: slide!(No.withPartial): Correctly handle first slide exhausting input range.

[FeepingCreature]

Apply the suggested change from https://issues.dlang.org/show_bug.cgi?id=19642

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19642	normal	std.range.slide!(No.withPartial) on lengthless forward range: get empty when expecting one window

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8738"

[quickfur]

This PR caused a regression: https://issues.dlang.org/show_bug.cgi?id=23976

[FeepingCreature]

Thanks! I'll have a look tomorrow.

[FeepingCreature]

Please see discussion at https://forum.dlang.org/post/ivmrolypalfsoqeqecki@forum.dlang.org : the specified behavior of slide is confusing and contradictory, and I don't understand what it's supposed to do, and when.