Initialise 'psa' to NULL on every code path in the Pageant client

[u/mdw/putty] / windows / website.url

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URL=http://www.chiark.greenend.org.uk/~sgtatham/putty/
\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0