Calling font-lock-add/remove-keywords after using the major mode in MuMaMo clears mumamo-eval-in-MAJOR-MODE

[tkf]

1. Open a buffer and enable MuMaMo mode with MAJOR-MODE.
2. Modify font-lock keywords for MAJOR-MODE using font-lock-remove-keywords and font-lock-add-keywords.
3. Open a buffer and enable the same MuMaMo mode with MAJOR-MODE.
4. You will get a lot of error like this in the *Messages* buffer:
   fun-var-sym=mumamo-eval-in-MAJOR-MODE, use-major-entry=(MAJOR-MODE (mumamo-eval-in-MAJOR-MODE mumamo-def-eval-in-MAJOR-MODE mumamo-kw-eval-in-MAJOR-MODE)
5. Evaluating mumamo-eval-in-MAJOR-MODE yields nil.

I checked with MAJOR-MODE=python-mode.

Original report:
https://github.com/tkf/emacs-ipython-notebook/issues/65