Resolve "Use of undefined values in SNS decoding due to wrong scope of array variable"

Closes #686 (closed)

[x] This issue happened in ivas_mdct_core_invQ function due to use-of-uninitialized-value. Fix made to address the issue. [x] After the initial fix, crash was observed in further frames in sns_interpolate_scalefactors_fx function. The buffer size is updated with which the issue is resolved.

MLD:
Original: 7 cases > 5
8.622672 9.193538 10.831529 10.852825 12.475984 19.9825 22.46314

+10dB: 23 cases > 5
5.663041 5.663041 6.289222 6.327793 6.992729 7.061021 7.341303 7.782776 7.858091 8.153455 10.755767 10.755767 11.334802 14.46254 15.203578 16.607782 16.9758 17.577759 19.374035 19.76078 23.576897 27.352047 38.927044

-10dB: 5 cases > 5
5.964953 6.009441 8.736943 14.835657 15.85611