[libunwind] [SEH] Clear DISPATCHER_CONTEXT when initializing a cursor

[mstorsjo]

Summary

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).

[phosek]

LGTM

libunwind/src/UnwindCursor.hpp

 UnwindCursor<A, R>::UnwindCursor(unw_context_t *context, A &as)
     : _addressSpace(as), _unwindInfoMissing(false) {
   static_assert((check_fit<UnwindCursor<A, R>, unw_cursor_t>::does_fit),
                 "UnwindCursor<> does not fit in unw_cursor_t");
   static_assert((alignof(UnwindCursor<A, R>) <= alignof(unw_cursor_t)),
                 "UnwindCursor<> requires more alignment than unw_cursor_t");
   memset(&_info, 0, sizeof(_info));
   memset(&_histTable, 0, sizeof(_histTable));
+  memset(&_dispContext, 0, sizeof(_dispContext));
   _dispContext.ContextRecord = &_msContext;
   _dispContext.HistoryTable = &_histTable;
   // Initialize MS context from ours.
   R r(context);
   RtlCaptureContext(&_msContext);
   _msContext.ContextFlags = CONTEXT_CONTROL|CONTEXT_INTEGER|CONTEXT_FLOATING_POINT;
 #if defined(_LIBUNWIND_TARGET_X86_64)
   _msContext.Rax = r.getRegister(UNW_X86_64_RAX);

 
 template <typename A, typename R>
 UnwindCursor<A, R>::UnwindCursor(CONTEXT *context, A &as)
     : _addressSpace(as), _unwindInfoMissing(false) {
   static_assert((check_fit<UnwindCursor<A, R>, unw_cursor_t>::does_fit),
                 "UnwindCursor<> does not fit in unw_cursor_t");
   memset(&_info, 0, sizeof(_info));
   memset(&_histTable, 0, sizeof(_histTable));
+  memset(&_dispContext, 0, sizeof(_dispContext));
   _dispContext.ContextRecord = &_msContext;
   _dispContext.HistoryTable = &_histTable;
   _msContext = *context;
 }
 
 
 template <typename A, typename R>
 bool UnwindCursor<A, R>::validReg(int regNum) {