Remove redundant null checks for arguments to sftp_{get,put}_file

which are (a) never NULL anyway, and (b) have already been
dereferenced by the time we make those checks so it would be too late
if they were.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9906 cda61777-01e9-0310-a592-d414129be87e