Fix empty union when TLS is disabled

[gilles-peskine-arm]

When all TLS 1.2 support is disabled, union mbedtls_ssl_premaster_secret was empty, which is not valid C even if the union is never used. Fixes #6628.

As this is a bit of an edge case and has a simple workaround (don't include mbedtls/ssl.h if you aren't going to use it), I haven't gone through the effort of adding a test. If we think this really needs a test, I think we should use the following strategy: have a test program that includes all headers, which is built by a plain make — this way we would test that all headers work correctly in all the configurations we test (including ones with no TLS, and ones with picky compilers). We already have such a test program, but it's a C++ program, which we don't build in most configurations. So the implementation of this test would be to change programs/test/generate_cpp_dummy_build.sh to generate a header that includes all the library headers, and separate out the trivial C++ wrapping and make a similar C wrapping that is always built. Then make sure we pass -pedantic in CFLAGS in all.sh (otherwise gcc doesn't mind).

Manual testing:

scripts/config.py crypto
make lib CFLAGS='-Werror -pedantic'

Before: “error: union has no members”; after: passes.

PR checklist

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

• changelog provided
• backport Backport 2.28: Fix empty union when TLS is disabled #7987
• tests manual: I don't think it's worth the effort to automate

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checklist for PR contributors.

[tom-cosgrove-arm]

LGTM

[irwir]

A slightly better hack for typical configurations - no extra byte.

#if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
unsigned char _pms_rsa[48]; /* RFC 5246 8.1.1 /
#else
unsigned char dummy; / Make the union non-empty even with SSL disabled */
#endif

[gilles-peskine-arm]

@irwir It's a union. There's an extra byte only if it would be empty otherwise, and that's the point.