bpo-36734: Fix compilation of faulthandler.c on HP-UX (GH-12970)

Initialize "stack_t current_stack" to zero using memset().

Misc/NEWS.d/next/Library/2019-04-26-17-14-20.bpo-36734.p2MaiN.rst

+Fix compilation of ``faulthandler.c`` on HP-UX. Initialize ``stack_t
+current_stack`` to zero using ``memset()``.

Modules/faulthandler.c

@@ -1370,7 +1370,8 @@ void _PyFaulthandler_Fini(void)
 #ifdef HAVE_SIGALTSTACK
     if (stack.ss_sp != NULL) {
         /* Fetch the current alt stack */
-        stack_t current_stack = {};
+        stack_t current_stack;
+        memset(&current_stack, 0, sizeof(current_stack));
         if (sigaltstack(NULL, &current_stack) == 0) {
             if (current_stack.ss_sp == stack.ss_sp) {
                 /* The current alt stack is the one that we installed.