flacdec: Add a check for small buffer size. This ensures reading as

flacdec: Add a check for small buffer size.  This ensures reading as
much of the frame header as possible without excluding the smallest
possible FLAC frame.  It also fixes a false positive warning message
that was being emitted at the end of decoding.

Originally committed as revision 17816 to svn://svn.ffmpeg.org/ffmpeg/trunk