Fix the line-resizing bug in scroll(). (Thanks to RDB for pointing

Fix the line-resizing bug in scroll(). (Thanks to RDB for pointing
it out. A line was removed from the scrollback, cleared, and placed
at the bottom of the screen. Fine, except that the clearing process
assumed the line was the right length already, and thanks to lazy
resizing this wasn't necessarily the case. Segfaults and memory
corruption ensued.)

git-svn-id: http://svn.tartarus.org/sgt/putty@1129 cda61777-01e9-0310-a592-d414129be87e