Fix a possible memset (clear memory) vulnerability

After a code review I got the following feedback, pointing out a possible
vulnerability when clearing memory:
The usage of the free() statement directly after a memset() statement. Such
sequence might be recognized and removed by a compiler in case of
optimisation.

I reviewed the ZRTP code and found some places that could be affected by this
compiler optimization. I fixed these memset calls using a specific call
seqeuence to avoid this sort of optimization.