Fix ASF format parser's broken UTF-16 string handling

1. Add a PUT_UTF8 macro to common.h; code borrowed from libavcodec/flacenc.c.
2. Make use of the macro in flacenc.c
Patch by Zuxy Meng % zuxy P meng A gmail P com %
Original thread:
Date: Nov 5, 2006 9:56 AM
Subject: [Ffmpeg-devel] PUT_UTF8 & asf format enhancement

Originally committed as revision 6911 to svn://svn.ffmpeg.org/ffmpeg/trunk