CLOS MOP bug in 1.8.0

[Mark Evenson]

Filed as [479][].  

Funny that this has popped up because the only change that I can find to
CLOS/MOP between 1.7.1 and 1.8.0 was in the CALL-NEXT-METHOD/NEXT-METHOD-P
implementation.  But this is why we test.  Or should test better.

Thanks for the explicit report with the test case!

[479]: <https://abcl.org/trac/ticket/479>

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