ssl_mail_client: Fix unbounded write of sprintf()

[szsam]

Description

These calls to sprintf may overflow buf because opt.mail_from and opt.mail_to are controlled by users. Fix by replacing sprintf with snprintf.

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
• [ ] backport Need to cherry-pick to mbedtls-2.28: #8907; 3.6: TODO
• [x] tests not required

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[gilles-peskine-arm]

Windows builds are failing.

[daverodgman]

From the CI:

281>C:\Windows\workspace\mbed-tls-pr-head_PR-8897-head\worktrees\tmpsowuty4p\programs\ssl\ssl_mail_client.c(730): warning C4013: 'snprintf' undefined; assuming extern returning int [c:\windows\workspace\mbed-tls-pr-head_PR-8897-head\worktrees\tmpsowuty4p\cmake_solution\programs\ssl\ssl_mail_client.vcxproj]

[szsam]

I replaced snprintf with mbedtls_snprintf and checked the return values of mbedtls_snprintf. If that looks good and passes CI, I will update the backport PR.

[minosgalanakis]

Hi, and thank you for your contribution. Could you please try to fix the code style?

It can be done automatically using the scripts/code_style.py -f

[szsam]

Can we start CI?

[ronald-cron-arm]

Can we start CI?

I've just done it.

[szsam]

Ready for review.

[mpg]

I'd say this needs a 3.6 backport too, now. Edit: ah, was already in the PR description, I had missed it as the end of the line after the 2.28 link.