fix #20912 lexing problem with #define // comments

[WalterBright]

The problem is the current position in the buffer gets advanced to the end when it encounters a 0 in a comment.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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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 + dmd#20914"