SongPage: fix assert() checking the "tag" range

AppendTag() is called with a mpd_tag_type, and therefore it should be
in that range.  The assert() has always been wrong, but that had no
effect until commit f64f116

Closes #149

NEWS

@@ -2,6 +2,7 @@ ncmpc 0.51 - not yet released
 * repaint main area after terminal was resized
 * pressing Enter while in "jump mode" activates selected item
 * fix crash in new text input dialog
+* fix crash on song info page
 
 ncmpc 0.50 - (2024-09-12)
 * build: require Meson 0.60

src/SongPage.cxx

@@ -289,7 +289,7 @@ SongPage::AppendTag(const struct mpd_song *song,
 	unsigned i = 0;
 	const char *value;
 
-	assert((unsigned)tag < std::size(tag_labels));
+	assert(tag < MPD_TAG_COUNT);
 	assert(label != nullptr);
 
 	while ((value = mpd_song_get_tag(song, tag, i++)) != nullptr)