enforce the following:
* Packet must have at least one byte of payload and four bytes of padding.
* Total packet length must not exceed 35000 bytes compressed.
* Total packet length including length field must be a multiple of cipher
block size (or eight bytes).
The feebleness of our old checks was noticed by Ben Rudiak-Gould.
git-svn-id: svn://svn.tartarus.org/sgt/putty@5981
cda61777-01e9-0310-a592-
d414129be87e