JSON: Fix memory leak in parser

- Fix JSON root object leaking out by using delete.
  This is by no means a comprehensive fix since exceptions are involved
  in a lot of places, but it does fix the memory leak in all cases
  checked in Taskwarrior's test suite.
- Going with a shared_ptr or unique_ptr here might be a better
  solution, but would involve more code changes.

src/Task.cpp

@@ -706,6 +706,8 @@ void Task::parseJSON (const std::string& line)
       }
     }
   }
+
+  delete root;
 }
 
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