ntdll: Only set the security cookie if it has not already been set.

[Erich E. Hoover]

The attached patch fixes Bug #38895 (Multiple applications fail to
load or crash due to incorrect security cookie randomization by
loader).  It's necessary to ensure that once the cookie has been
established that it does not change throughout the course of
execution, so this patch makes sure we only set the cookie value if
it's not set to the default value.

Looked over and tested by André Hentschel on ARM64.
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