fix issue 20949 - std.range.popFront is unsafe in release mode

[aG0aep6G]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20949	normal	std.range.popFront is unsafe in release mode

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#7535"

[aG0aep6G]

I don't know where to add a test for this. The issue only manifests when the assert in popFront is omitted. Apparently, -unittest overrides -release and keeps the assert. So a unittest doesn't cut it. Do we have a place in Phobos for testing a complete program?