Fix a few compiler warnings

There was one uninitialized value warning and two mismatched-sign
compare warnings. They both appear valid, the first occurring when MPD's
"format-stopped" contains {songPosition} or {queueLength} and the second
occurring when the clock's "timezones" array is more than 2 billion
items long (not likely, I admit). Fix both issues.

src/modules/mpd/mpd.cpp

@@ -100,7 +100,7 @@ void waybar::modules::MPD::setLabel() {
   auto format = format_;
 
   std::string          artist, album_artist, album, title, date;
-  int                  song_pos, queue_length;
+  int                  song_pos = 0, queue_length = 0;
   std::chrono::seconds elapsedTime, totalTime;
 
   std::string stateIcon = "";