A user points out that in commit 6143a50ed228fdf, when I converted all
use of the registry to functions that return a newly allocated buffer
instead of allocating a buffer themselves beforehand, I overlooked
that one use of the old idiom was reusing the preallocated buffer as
work space.
I _hope_ nobody still needs this code - the 'old-style' host key cache
format it handles was replaced in 2000. If anyone has a PuTTY host key
cache entry that's survived 22 years without either having to be
reinitialised on a new system or changed when the machine's host key
was upgraded, they're doing better than I am!
But if it's still here, it should still work, obviously. Replaced the
reused buffer with a strbuf, which is more robust anyway.
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