Fix potential crashes due to timing

mods/ambiance/init.lua

@@ -74,6 +74,10 @@ if minetest.settings:get_bool("ambiance_enable") == true then
 
          for soundname, sound in pairs(ambiance.sounds) do
             if not minetest.settings:get_bool("ambiance_disable_" .. soundname) then
+               if lastsound[name] == nil then
+                  -- lastsound is not initialized yet
+                  return
+               end
                if lastsound[name][soundname] then
                   lastsound[name][soundname] = lastsound[name][soundname] + dtime
                else

mods/player_effects/init.lua

@@ -163,6 +163,10 @@ local function step(dtime)
 
    local gt = minetest.get_gametime()
 
+   if player_effects.effects == nil then
+      return
+   end
+
    for _, player in pairs(minetest.get_connected_players()) do
       local name = player:get_player_name()