Avoid implementation defined behavior in a test.

[danalbert]

Summary

num_put::put uses %p for pointer types, but the exact format of %p is
implementation defined behavior for the C library. Compare output to
snprintf for portability.

test/std/localization/locale.categories/category.numeric/locale.nm.put/facet.num.put.members/put_pointer.pass.cpp

 {
     const my_facet f(1);
     {
         std::ios ios(0);
         void* v = 0;
         char str[50];
         output_iterator<char*> iter = f.put(output_iterator<char*>(str), ios, '*', v);
         std::string ex(str, iter.base());
-        assert(ex == "0x0" || ex == "(nil)");
+        char expected_str[32] = {};
+        // num_put::put uses %p for pointer types, but the exact format of %p is
+        // implementation defined behavior for the C library. Compare output to
+        // snprintf for portability.
+        int rc = snprintf(expected_str, sizeof(expected_str), "%p", v);
+        assert(rc > 0);
+        assert(ex == expected_str);
     }
 }