Modify bindings to enforce that an error handler callback is only set after binding to a msg pipe.

mojo/public/cpp/bindings/lib/binding_state.h

Index: mojo/public/cpp/bindings/lib/binding_state.h
diff --git a/mojo/public/cpp/bindings/lib/binding_state.h b/mojo/public/cpp/bindings/lib/binding_state.h
index 82b1a327b21a13d318f9bf82d3de6e4c473472d0..0fff8e332b16d3d19f7fa5b8ca2eb9589ab1a02f 100644
--- a/mojo/public/cpp/bindings/lib/binding_state.h
+++ b/mojo/public/cpp/bindings/lib/binding_state.h
@@ -89,6 +89,7 @@ class BindingState<Interface, false> {
   }
 
   void set_connection_error_handler(const Closure& error_handler) {
+    DCHECK(is_bound());
     connection_error_handler_ = error_handler;
   }
 
@@ -113,6 +114,7 @@ class BindingState<Interface, false> {
     router_->set_connection_error_handler(Closure());
     delete router_;
     router_ = nullptr;
+    connection_error_handler_.reset();
   }
 
   internal::Router* router_ = nullptr;