Add extra check and warning on missing player data.

* For some unexplained reason, `mcl_burning.storage[player]` can
  sometimes be `nil`, causing a crash in `on_leaveplayer()`.  This
  commit adds a check for that.  If a `nil` is encountered, a sane
  value is substituted and a warning is set to the log.

mods/ENTITIES/mcl_burning/init.lua

@@ -67,6 +67,13 @@ end)
 
 local function on_leaveplayer(player)
 	local storage = mcl_burning.storage[player]
+	if not storage then
+		-- For some unexplained reasons, mcl_burning.storage can be `nil` here.
+		-- Logging this exception to assist in finding the cause of this.
+		minetest.log("warning", "on_leaveplayer: missing mcl_burning.storage "
+				.. "for player " .. player.name)
+		storage = {}
+	end
 	storage.fire_hud_id = nil
 	player:get_meta():set_string("mcl_burning:data", minetest.serialize(storage))
 	mcl_burning.storage[player] = nil