Fix a potential crash in ssh_setup_portfwd.

Fix a potential crash in ssh_setup_portfwd.

If we search for a colon by computing ptr + host_strcspn(ptr,":"),
then the resulting pointer is always non-NULL, and the 'not found'
condition is not !p but !*p.

This typo could have caused PuTTY to overrun a string, but not in a
security-bug sense because any such string would have to have been
loaded from the configuration rather than received from a hostile
source.

git-svn-id: http://svn.tartarus.org/sgt/putty@10123 cda61777-01e9-0310-a592-d414129be87e