Occasional seg fault in make check at dispatch.cc

[John W. Eaton]

Thanks for the stack traces.

Does the following change fix the problem for you (at least, does it
avoid the segfault)?

  http://hg.savannah.gnu.org/hgweb/octave/rev/7838271ee25c

I think this change makes the intent clearer, but I still don't see
anything wrong with the previous code.  But I can be kind of stupid
about recursion sometimes, so maybe I'm missing something.

jwe