Changes
   * mbedtls_mpi_read_binary(), mbedtls_mpi_read_binary_le() and
     mbedtls_mpi_read_string() now construct an mbedtls_mpi object with 0 limbs
     when their input has length 0. Note that this is an implementation detail
     and can change at any time, so this change should be transparent, but it
     may result in mbedtls_mpi_write_binary() or mbedtls_mpi_write_string()
     now writing an empty string where it previously wrote one or more
     zero digits when operating from values constructed with an mpi_read
     function and some mpi operations.