Fix Issue 21256 - Segfault with Appender!string.init.toString()

[e10s]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21256	critical	Segfault with Appender!string.init.toString()

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 "stable + phobos#7691"

[thewilsonator]

Two things: please update the commit message to match the copyedited issue title and please rebase this PR to stable.

[thewilsonator]

Hmm, please force-push to restart the CI.