advapi32: increase robustness for incorrect api usage

[Michael Jung]

The attached patch fixes the advapi32 related crash in the "Uru - Ages Beyond 
Myst Demo" installer.

When a context is released, the corresponding heap based struct variable is 
zero'ed before being released. When the CryptDestroyHash function is called, 
the corresponding context is checked if it was already being zero'ed out. 
Note that a heap location is read, which has already been released. However, 
this only happens in case of incorrect api usage. This is not guaranteed to 
work, but it should be more robust.

I suspect that windows does it in a similar way. Looking at the actual values 
of HCRYPTHASH variables shows that those are addresses in WinXP SP2. 
Considering this, I think that the only 100% clean way to implement this 
behaviour would be to have a list of currently existing hash and key objects 
in the context, which then would be marked invalid when the context is 
released.

If this patch is applied, I will look into advapi32 to find similar places 
where this scheme should be implemented. If the patch is rejected, it would 
be nice to get some feedback on how people think it should be done correctly.

Changelog:
 Make advapi32 more robust in case of incorrect api usage

Bye,
-- 
Michael Jung
mjung at iss.tu-darmstadt.de
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