plugins: osc ugens - fix GET_TABLE macro

sign matters. fixes #722

Signed-off-by: Tim Blechmann <tim@klingt.org>
(cherry picked from commit af38f02314c36a3f6e3e5b6187b13b4b7de1d760)
plugins: OscUGens - ensure buffer initialization

Signed-off-by: Tim Blechmann <tim@klingt.org>
(cherry picked from commit b3b0948f4d4f90e7802e7f5e83dfeda175aca850)