We only initialize a few fields in DISPATCHER_CONTEXT - don't leave the rest in an uninitialized state; make sure the whole struct is in a deterministic state. This makes nondeterministic failures deterministic, for some cases relating to forced unwinding on aarch64/arm (which requires filling in parsing of the xdata for finding the exception handler and LSDA). Differential Revision: https://reviews.llvm.org/D148660