Add missing checks in update_for_intended_size() in the font selector

Add missing checks in update_for_intended_size() in the font selector
code, which would have coped badly if ever asked to select the first
font in the list at a size smaller than it supported. Luckily the
first font tended to be one of the X numeric aliases (e.g. 10x20)
which was stored with size zero, so this probably didn't actually come
up for anyone, but better safe than sorry.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9910 cda61777-01e9-0310-a592-d414129be87e