Jordan Russell's patch (again): a couple of registry read operations

Jordan Russell's patch (again): a couple of registry read operations
were using RegCreateKey instead of RegOpenKey by mistake. This also
required a fix in settings.c to deal gracefully with a NULL return
from enum_settings_start() - since the use of RCK had caused this
never to happen, the code path had never been tested.

git-svn-id: http://svn.tartarus.org/sgt/putty@1516 cda61777-01e9-0310-a592-d414129be87e