avcodec/sbrdsp_fixed: Fix assertion failure in sbr_sum_square_c()

This also increases the range of input values supported as well as
decreasing the operation dependencies in the main loop, improving
speed on modern CPUs.

Fixes part of: 2045/clusterfuzz-testcase-minimized-6751255865065472

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
