Makes it possible to do a "dry run" generation for in-place padding

Michael Klingbeil authored 1 year ago and Jean-Marc Valin committed 1 year ago

Add unit tests for opus_packet_extensions_count, opus_packet_extensions_parse,
and opus_packet_extensions_generate. Add various assertions for NULL buffers or
negative length arguments. Add explicit check for invalid length in
opus_packet_extensions_count. Check for extension id >127 in
opus_packet_extensions_generate. Check for invalid length input in
opus_packet_extensions_generate.

Signed-off-by: Jean-Marc Valin <jmvalin@amazon.com>

Now generating the extension in place once all the data is already
in the right place.