Fixes an assertion failure in SILK

We stop the schur recursion before any reflection coefficient
goes outside of ]-1,1[ and we force reporting a residual energy
of at least 1.
Assertion was:
Fatal (internal) error in ../silk/fixed/noise_shape_analysis_FIX.c, line 290: assertion failed: nrg >= 0
triggered by:
opus_demo voip 16000 1 12500 -bandwidth WB -complexity 10 pl04f087.stp-crash out.pcm