Fix bug in taglib txxx handling

Fix bug in taglib txxx handling

Currently, vlc puts the value *and* the description in the value field
for id3 TXXX tags, e.g. in Media Information:
REPLAYGAIN_TRACK_GAIN: REPLAYGAIN_TRACK_GAIN +3.30 dB

This leads to vlc_audio_replay_gain_MergeFromMeta() failing because it
can't parse the floating point value. Since the fieldList() of an
UserTextIdentificationFrame has exactly two members, it is safe to just
use the last (value) member. In Media Information:
REPLAYGAIN_TRACK_GAIN: +3.30 dB

This makes ReplayGain work for mp3 files.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>