This patch fixes various memory leaks found with ASAN.
  - av_dict_free wasn't called after av_dict_set
  - avformat_free_context wasn't called
  - safe_by_default: all av_malloc has been replaced by std::vector

Tuleap: #412
Change-Id: Ic47cc87b8ed013e7fac5dea80ea7e7aee96139b7