Fix for use-of-uninitialized-value issue 686

[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.