Backport 2.28: ssl_mail_client: Fix unbounded write of sprintf()

[szsam]

Description

This is the 2.28 backport of #8897. The backport is trivial.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [x] changelog not required
• [x] backport of #8897
• [x] tests not required

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