ChangeLog.d/mpi-add-0-ub.txt

Bugfix
   * Fix undefined behavior (typically harmless in practice) of
     mbedtls_mpi_add_mpi(), mbedtls_mpi_add_abs() and mbedtls_mpi_add_int()
     when both operands are 0 and the left operand is represented with 0 limbs.