Fix build warning in Audio.cpp

\interface\src\Audio.cpp(309): warning C4018: '<' : signed/unsigned mismatch
Merge pull request #3015 from ctrlaltdavid/build_warning_fix

Fix build warning in Audio.cpp
fix reference to attribute in STUN packet
Merge pull request #3017 from birarda/master

fix offset to attribute in STUN packet
Merge branch 'master' of https://github.com/highfidelity/hifi