Rework the new type-check in sresize so that it doesn't cause a

compile warning ('left-hand operand of comma expression has no
effect'), which of course becomes fatal under -Werror.

(This would have been instantly noticeable to people compiling with
the old-fashioned Makefile.gtk, which does include -Wall -Werror, but
those of us using the new autoconf makefile hadn't noticed.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9582 cda61777-01e9-0310-a592-d414129be87e