Common: fix variable used after null check

Warning from Coverity Scan.
Common: fix out-of-bounds access in setters functions of ParamSet

* Avoid const_cast.
* Same way in all AvTranscoder to get the FFmpeg errors by using an
array fo char passed to av_strerror function.
* Warnings from Coverity Scan.
VideoStreamProperties: fix uninitialized variable topFieldFirst

Warning from Coverity Scan.
Merge pull request #64 from cchampet/fix_coverity_warnings

Fix Coverity Scan warnings