1.0.30.26: fix failing AVER in CONVERT-MV-CALL

1.0.30.26: fix failing AVER in CONVERT-MV-CALL

 See https://bugs.launchpad.net/sbcl/+bug/392203

 Patch by Larry D'Anna.

 His comments:

   Ever since Spice Lisp, convert-mv-call had returned without doing
   anything if (functional-entry-fun fun) is not null, ie if fun
   possesses a XEP. 0.8.5.5 replaces this criterion with "if the last
   optional entry has references", and signals an error if the last
   optional entry has no references and the XEP exists.

   I can't know exactly what Alexy was thinking when he put the aver
   in, but I can guess: If the XEP exists it should contain a
   reference to the last entry point, so if the last entry point has
   no refs and a XEP exists something went wrong. However, if the
   number of required + optional arguments is 0, then XEP doesn't need
   the "last" entry point, it can always use the "more" entry point
   instead, which is exactly what seems to have happened in this case.

   This patch combines the two conditions. convert-mv-call will return
   without action if *either* a XEP exists, *or* the last optional
   entry has references."