Init two pointers in const exercise.

Although the exercise is mostly about battling with the compiler, once
people finish this work, they will encounter a crash when running the
program.
Here, we init the two pointers to &a and &b to create a runnable
program.

exercises/constness/constplay.cpp

@@ -63,8 +63,8 @@ int main() {
 
     // try constant arguments of functions with pointers
     {
-      int *p = 0;
-      const int *r = 0;
+      int *p = &a;
+      const int *r = &b;
       write(p);
       write(r);
       read(p);