Remove a redundant while-loop condition when reading RFC822-style

Remove a redundant while-loop condition when reading RFC822-style
header text from a PuTTY key file.

(It's silly to have both while (len > 0) at the top of the loop _and_
an if (len == 0) return in the middle, and in fact the former was the
erroneous one since it would have prohibited a 39-character header,
which I intended to be permitted.)

git-svn-id: http://svn.tartarus.org/sgt/putty@9926 cda61777-01e9-0310-a592-d414129be87e