Extra parameter for the output buffer size

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.