Extra parameter for the output buffer size
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The following functions now take an extra parameter indicating the size of the output buffer:

* `mbedtls_ecdsa_write_signature()`, `mbedtls_ecdsa_write_signature_restartable()`
* `mbedtls_pk_sign()`, `mbedtls_pk_sign_restartable()`

The requirements for the output buffer have not changed, but passing a buffer that is too small now reliably causes the functions to return an error, rather than overflowing the buffer.