Sanity check to avoid crash when pjsip returns negatives packets size. This can occurs when media multithreading is enabled.
Merge pull request #5 from r3gis3r/pr_multithread

Sanity check to avoid crash when pjsip returns negatives packets size.
Fix NULL pointer dereference if userCallback was not set
Merge pull request #6 from saghul/usercallback-crash

Fix NULL pointer dereference if userCallback was not set
Fix Makefile to support PJSIP 2
Fix problems with C compiler and MS C-compiler
Fix maximum buffer length, change handling of user callback structure

Reported and proposed by Eeri Kask, TU Dresden
(Eeri.Kask@mailbox.tu-dresden.de)
Merge branch 'develop' of github.com:wernerd/ZRTP4PJ into develop