Fix a bug in RTCP packet reception which caused heap corrupted by overwriting

There were some errors in processing a compound RTCP packet which contains more than one RTCP type. 
This caused Asterisk crash when receiving some compound RTCP packets because the heap was corrupted 
by overwriting the dynamically allocated buffer.

Also with the following changes.

- Define AST_BUILDOPT_SUM as "" to avoid loading error as below:
  Module 'chan_voicemngr.so' was not compiled with the same compile-time options as this version of
  Asterisk.
- Update module description in AST_MODULE_INFO()
- Remove some unused macros
- Some coding style improvements