Fix for issue 684
[x] Increasing Synthesis_STFT_fx guard bits for crash issue [x] get_next_index_4_by_15 table value supports Word16. Last entry changed from 32768 to 32767. [x] coherence_flt value updation logic changed.
[x] Increasing Synthesis_STFT_fx guard bits for crash issue [x] get_next_index_4_by_15 table value supports Word16. Last entry changed from 32768 to 32767. [x] coherence_flt value updation logic changed.