Don't call KiUserExceptionDispatcher directly on the stack pointer
stored in the CONTEXT, but use the one stored in syscall_frame
(which includes the stack allocation in e.g. RtlRaiseException).

This fixes unwinding test cases that worked before
08c4419a.
Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>