Fix crash when using ffmpeg to play mp3 and ogg

Fix crash when using ffmpeg to play mp3 and ogg

	* src/media/media-ffmpeg.c (media_ffmpeg_analyse_audio): Refactor
	detection of sample size using av_get_bytes_per_sample. Error when
	format is not supported instead of silently go ahead and possibly
	crash later.

	* src/media/media.c (sxe_msf_DBL_up): Implement conversion to
	internal format from double.
	(sxe_msf_DBL_down): Implement from internal format to double.

	* src/media/media.h: Declare double encoded samples.

	* m4/sxe-mm.m4 (SXE_CHECK_FFMPEG_LIBS): Detect
	av_get_bytes_per_sample and av_get_sample_fmt_string.
	(SXE_MM_CHECK_FFMPEG): Enforce presenc of av_get_bytes_per_sample
	and av_get_sample_fmt_string.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>