Remove redundant check for NULL in sshfwd_close(). The thing we're

testing against NULL has already been dereferenced by the time we
bother to test it, so it's a bit pointless - and in any case, no
null pointer can come to this function from any existing call site.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8990 cda61777-01e9-0310-a592-d414129be87e