Possible saturation in stereo FER

With cc43b322 and this bitstream : es02_44s.bit_error.32000

A crash happen when rescaling a memory in the TDBWE:

    /* rescale the TBE post proc memory */
    FOR( i = 0; i < LPC_SHB_ORDER; i++ )
    {
        hBWE_TD->mem_stp_swb_fx[i] = shl( hBWE_TD->mem_stp_swb_fx[i], sub( Q_bwe_exc, st->prev_Q_bwe_syn ) );
        move16();
    }