ext/ffmpeg/: Free strings atleast when finalizing elements.

ext/ffmpeg/: Free strings atleast when finalizing elements.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcfg.c: (gst_ffmpeg_cfg_install_property),
(gst_ffmpeg_cfg_finalize):
* ext/ffmpeg/gstffmpegcfg.h:
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_finalize),
(ffmpegenc_setup_working_buf), (gst_ffmpegenc_chain_video),
(gst_ffmpegenc_flush_buffers):
Free strings atleast when finalizing elements.
* tests/check/generic/libavcodec-locking.c: (GST_START_TEST),
(simple_launch_lines_suite):
Fix some leaks.