Fix Lua errors when the client is built without the sound framework (#1142)

modules/client/client.lua

@@ -1,5 +1,8 @@
 local musicFilename = "/sounds/startup"
-local musicChannel = g_sounds.getChannel(1)
+local musicChannel = nil
+if g_sounds then
+  musicChannel = g_sounds.getChannel(SoundChannels.Music)
+end
 
 function setMusic(filename)
   musicFilename = filename
@@ -27,12 +30,14 @@ end
 
 function startup()
   -- Play startup music (The Silver Tree, by Mattias Westlund)
-  musicChannel:enqueue(musicFilename, 3)
-  connect(g_game, { onGameStart = function() musicChannel:stop(3) end })
-  connect(g_game, { onGameEnd = function()
-      g_sounds.stopAll()
-      musicChannel:enqueue(musicFilename, 3)
-  end })
+  if musicChannel then
+    musicChannel:enqueue(musicFilename, 3)
+    connect(g_game, { onGameStart = function() musicChannel:stop(3) end })
+    connect(g_game, { onGameEnd = function()
+        g_sounds.stopAll()
+        musicChannel:enqueue(musicFilename, 3)
+    end })
+  end
 
   -- Check for startup errors
   local errtitle = nil
@@ -57,7 +62,9 @@ function init()
                    onExit = exit })
 
   g_window.setMinimumSize({ width = 600, height = 480 })
-  g_sounds.preload(musicFilename)
+  if musicChannel then
+    g_sounds.preload(musicFilename)
+  end
 
   -- initialize in fullscreen mode on mobile devices
   if g_window.getPlatformType() == "X11-EGL" then