Tighten up a lot of casts from unsigned to int which are read by one

Tighten up a lot of casts from unsigned to int which are read by one
of the GET_32BIT macros and then used as length fields. Missing bounds
checks against zero have been added, and also I've introduced a helper
function toint() which casts from unsigned to int in such a way as to
avoid C undefined behaviour, since I'm not sure I trust compilers any
more to do the obviously sensible thing.

git-svn-id: http://svn.tartarus.org/sgt/putty@9918 cda61777-01e9-0310-a592-d414129be87e