Add precondition on call_stackt::pop

Guarantee that it is never called on an empty stack.

Author: romainbrenguier

src/goto-symex/call_stack.h

@@ -34,6 +34,7 @@ class call_stackt : public std::vector<framet>
 
   void pop()
   {
+    PRECONDITION(!empty());
     pop_back();
   }