Alpha 6.3.3 Patch

Another small patch to fix a reported issue.

Some people have encountered a problem where their defenders were taking damage right at the start of the wave. I have been unable to reproduce this issue but this patch should give defenders a short period of invulnerability at the start of each wave and hopefully take care of the issue.

If you continue to encounter the problem after this patch please send in a report so I can take another look at it.