Do not attempt to disconnect when a packet checksum is invalid

Formerly receiving a packet with an invalid checksum would cause a disconnection, which is not desirable because:

• invalid checksums, albeit uncommon, are a normal occurence with UDP
• they typically happen on a individual, occasional isolated packet and thus do not disrupt netgames
• just calling Net_CloseConnection() is not enough to disconnect completely and instead causes weird glitches, such as being able to play but not being able to use the chat.