fix issue 15876 - various cases of SEGFAULT when formatting parser errors

[ghost]

Just added some checks before the unsafe accesses pointed by GDB when running the test cases.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	15876	normal	various cases of SEGFAULT when formatting parser errors

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 + dmd#9007"