mbedtls-2.28: Fix llvm error: variable 'default_iv_length' may be used uninitialized

[sergio-nsk]

Description

Backport of #7210

mbedtls/library/psa_crypto.c:3428:30: error: variable 'default_iv_length' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
        memcpy(iv, local_iv, default_iv_length);
                             ^~~~~~~~~~~~~~~~~

The warning is false positive, but treated as error and prevents compiling on Windows with LLVM Clang.

Gatekeeper checklist

• changelog not required
• backport done
• tests not required

[sergio-nsk]

What is the issue with accepting this PR with the tiny change?
We would like to use mbedtls from the vendor upstream in git submodules, and w/o this change, we have to use a clone with patches.
Compilers are not smart enough to deduce status == PSA_SUCCESS is false and they raise the warning: variable 'default_iv_length' may be uninitialized when used here: memcpy(iv, local_iv, default_iv_length).

[tom-cosgrove-arm]

LGTM

[davidhorstmann-arm]

LGTM