Add a bounds check in the word-by-word selection code to prevent

Add a bounds check in the word-by-word selection code to prevent
attempting to call lineptr() with a y-coordinate off the bottom of the
screen and triggering the dreaded 'line==NULL' message box.

This crash can only occur if the bottommost line of the screen has the
LATTR_WRAPPED flag set, which as far as I can see you can only
contrive by constructing a LATTR_WRAPPED line further up the screen
and then moving it down using an insert-line escape sequence. That's
probably why this bug has been around forever without anyone coming
across it.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9726 cda61777-01e9-0310-a592-d414129be87e