cygwin: initialize variable for stack unwinding

The third argument of RtlLookupFunctionEntry actually is documented as
_Inout_opt_ for both x64 and ARM, although generic doc says _Out_ only.

* exceptions.cc (__unwind_single_frame): Initialize hist variable.

winsup/cygwin/exceptions.cc

@@ -280,7 +280,7 @@ __unwind_single_frame (PCONTEXT ctx)
 {
   PRUNTIME_FUNCTION f;
   ULONG64 imagebase;
-  UNWIND_HISTORY_TABLE hist;
+  UNWIND_HISTORY_TABLE hist = {0};
   DWORD64 establisher;
   PVOID hdl;