Crashes when opening an encrypted mail

[MartinDietze]

This may be a duplicate of #241, but I am not sure. After receiving an eMail with some particular properties, mulberry will always crash when trying to open that mail. The crash happens in CSecurityPluginDLL::ToPath? in CSEcurityPluginDLL.cp; a check against NULL is missing for the argument, see the attached patch for details.

I've appended a sample mailbox with which the error is reproducable and a patch which will fix the immediate problem. I believe that the patch fixes a bug, however the reason for the crash we have here may be somewhere deeper in the code (why is the argument to ToPath?() NULL?).