Fix an EOF-testing goof in winhandl.c.

Fix an EOF-testing goof in winhandl.c.

I was having a play with clang's MSVC compatibility mode, just to see
how much of PuTTY it could compile, and one of its warnings pointed
out this error which must have crept in when I was changing the EOF
flags in winhandl.c from booleans to three-state enums - I left the !
on the front of what was previously an if (!thing) and needed to turn
into if (thing == EOF_NO).