Don't crash on buggy time calculation in time_over

internal.lua

@@ -289,8 +289,17 @@ function adv_spawning.time_over(minimum)
 	local now = adv_spawning.gettime()
 
 	local time_passed = now - adv_spawning.quota_starttime
-
-	assert(time_passed >= 0)
+	
+	if (time_passed < 0) then
+		if adv_spawning.timebackwardwarning ~= true then
+			core.log("error", "ADV_SPAWNING: Error either there's a bug in time"
+				.." calculation\n or your time just went backwards: old timestamp: "
+				.. adv_spawning.quota_starttime .. " current_time: " .. now .. "\n")
+			adv_spawning.timebackwardwarning = true
+			
+		end
+		return true
+	end
 
 	return (adv_spawning.quota_left - time_passed) < minimum
 end