decoder/opus: fix "readpicture" on Opus files

Don't return early from ScanOpusTags() if only
TagHandler::WantPicture() is set.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1413

src/decoder/plugins/OpusTags.cxx

@@ -91,7 +91,8 @@ ScanOpusTags(const void *data, size_t size,
 	if (!r.Expect("OpusTags", 8))
 		return false;
 
-	if (!handler.WantPair() && !handler.WantTag())
+	if (!handler.WantPair() && !handler.WantTag() &&
+	    !handler.WantPicture())
 		return true;
 
 	if (!r.SkipString())