If SEH is used in a C trap handler, the exception frame will be
registered before the call to KiEnterTrap, which means we save
the wrong trap handler. We'll therefore also restore this wrong
frame for the excepting code, resulting in a stale SEH chain.
We avoid this problem by saving the handler in the assembly
trap entry code instead of from C. While SEH in a C trap handler
should now theoretically be safe, we still forbid it through
asserts in the C KiEnterTrap variants to make any potential
future problems more obvious. Should this functionality be
needed at some point and deemed safe, these asserts can then be
removed.
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