Added another failing example.

* pointers to separate regions should be unequal.

examples/failing/extern_mem2.c

@@ -0,0 +1,19 @@
+#include <stdlib.h>
+#include <smack.h>
+
+void foo(int*);
+int* bar();
+
+int main() {
+  int *x = malloc(4);
+  int *y = malloc(4);
+  int *z;
+
+  foo(y);
+  z = bar();
+
+  *x = 1;
+  *z = 2;
+
+  __SMACK_assert(x != z);
+}