bugfix: delete definition

Broken since commit fde9e03370 on Apr 30. Now I have a couple more
manual tests.

0056-delete_definition

@@ -11,6 +11,9 @@ delete_definition = function(name)
 		if live.receive_run_time_error_from_app() then
 			break
 		end
+		if response_string then
+			break
+		end
 		love.timer.sleep(0.001)
 	end
 	Manifest_navigator.reload = true

Manual_tests.md

@@ -76,6 +76,18 @@ robust to apps changing out from under it.
   Switch back to the driver. Hit F4. The driver hangs and needs to be
   force-quit. [This is not ideal, but how things are today.]
 
+* press C-n, add an empty definition:
+
+    ```
+    foo = function()
+    end
+    ```
+  Press C-d, select `foo` to delete its definition.
+  Driver doesn't hang, no errors in logs.
+
+* press C-d, select `on` to try to delete its definition.
+  Driver shows an error that it's not allowed.
+
 ### Other compromises
 
 Lua is dynamically typed. Tests can't patch over lack of type-checking.