Fix issue 19740 - Incorrect result of BigInt * BigInt.

[shove70]

Fix issue 19740.

[dlang-bot]

Thanks for your pull request and interest in making D better, @shove70! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#6972"

[thewilsonator]

Please make sure the commit message contains "Fix issue 19740" so that its is picked up by the bot.

[n8sh]

@thewilsonator the bot didn't pick this up because it looks at commit messages rather than PR titles. At this point what's the the right thing to make the bug get closed and the fix noted in the changelog?

[thewilsonator]

Add another PR that has the correct commit message and provide a link to this PR from it, or add a changelog entry directly.