Jacob reports a segfault when using HTTP proxying under Minefield.

Jacob reports a segfault when using HTTP proxying under Minefield.
It appears that this is because Visual C's sscanf works by first
calling strlen to get the length of the string, so that its internal
read-character routine can be sure of never overrunning the buffer.
Quite why the internal read-char routine can't detect \0 _itself_
rather than having to have it found for it in advance I have no
idea. Sigh.

git-svn-id: http://svn.tartarus.org/sgt/putty@3844 cda61777-01e9-0310-a592-d414129be87e