Failsafe for all-zero LSF input features in Synthesis

[ljuvela]

If LSF type input features within a frame are all zero, the Synthesis program should interpret this as a neutral filter (and maybe give a warning).

All-zero LSFs commonly arise from zero-padding during modeling. Although they are not valid LSFs, they should be treated gracefully. Currently, Synthesis tries to fix the LSFs, which results in sharply resonant filters and loud outputs.