Revert flvdec.c part of r20836

	Log:
	Use AV_METADATA_DONT_STRDUP* / use av_malloced metadata instead of strduped
	arrays of fixed length.
	Code from ffmbc with changes to adapt to our metadata API.
Reason: memleak & fix is not trivial

Originally committed as revision 20866 to svn://svn.ffmpeg.org/ffmpeg/trunk