Fix for 3GPP issue 921: Decoder crash for MC51 at 48kbps FER in decoder_tcx_noisefilling_fx()

[x] Fix - Exponents for E_2ndlast and E_last not handled properly in
decoder_tcx_noisefilling_fx. BASOP_Util_Add_Mant32Exp also giving issues
when mant is 0 but exp is non-zero. Used Word64 to accumulate energies
and corrected exponent usage.