ext/ffmpeg/: Latest ffmpeg revision's avcodec_close frees more; use safer coding to prevent double free and other rel...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcfg.c: (gst_ffmpeg_cfg_fill_context):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_getcaps):
Latest ffmpeg revision's avcodec_close frees more; use safer coding to
prevent double free and other related segfaults.