Xiph metadata: fix bug causing redudant "extra" metadata
The IF_EXTRACT_FMT macro is to be used in a if-else-tree, the usage of
IF_EXTRACT prior to the following if-statement did however cause the
single if-else-tree to be split up into two (causing the fallback-else
at the end to be unconditionally hit, even if another branch had already
handled the data).
If "TITLE=" metadata was found, we would still end up inside the
fallback since the previous implementation of IF_EXTRACT_FMT would start
a new if-else-tree.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>