Previously there was no way to tell if an incoming packet was
incorrectly formatted. The new function SInPacket.isValid()
can be called after decryption/decompression to determine if
the packet wasn't decrypted and/or decompressed properly.
* Added "valid" attribute to SInPacket.
* Decryption routine unsets "valid" if input buffer is not divisible by 16.
* Decompression routine unsets "valid" on receiving zlib error.
rna88