This adds a leak, but fixes a crash. The leaking code is:

  "{0:.{precision}s}".format('hello world', precision=5)
I pretty sure it's because of the 'precision' keyword.
Still need to investigate further.

Objects/stringlib/string_format.h

@@ -416,6 +416,7 @@ get_field_object(SubString *input, PyObject *args, PyObject *kwargs)
             Py_DECREF(key);
             goto error;
         }
+        Py_INCREF(obj);
     } else {
         /* look up in args */
         obj = PySequence_GetItem(args, index);