Fix serverbrowser crash on malformed packet

[ppebb]

Server browser's SendMessageRequest handler does not have appropriate length guards. A packet of

bytes := []byte{
	0x00, 0x08, // Size
	0x02,                      // SendMessage Request
	0x4, 0x5, 0x6, 0x07, 0x08, // Body.
}

Will cause a panic.

This also adds panic recovery to server browser so other missed issues cannot take the server down.