Comment 0 for bug 1761950

(defun test ()
  (define-condition g638 () ())
  (setf (find-class 'g638) nil))

Produces:

Execution of a form compiled with errors.
Form:
  (SB-IMPL::%DEFUN 'TEST
                 (SB-INT:NAMED-LAMBDA TEST
                     NIL
                   (BLOCK TEST
                     (DEFINE-CONDITION G638
                         NIL
                         NIL)
                     (SETF (FIND-CLASS 'G638) NIL))))
Compile-time error:
  attempt to dump reference to obsolete class: #<SB-KERNEL::UNDEFINED-CLASSOID G638>
   [Condition of type SB-INT:COMPILED-PROGRAM-ERROR]

Restarts:
 0: [ABORT] Abort compilation.
 1: [*ABORT] Return to SLIME's top level.
 2: [ABORT] abort thread (#<THREAD "worker" RUNNING {1002D46033}>)

Backtrace:
  0: (SB-FASL::LOAD-FASL-GROUP #S(SB-FASL::FASL-INPUT :STREAM #<SB-SYS:FD-STREAM for "file /tmp/slimebA9Tm4.fasl" {1002F596A3}> :TABLE #(22 #<PACKAGE "SB-IMPL"> SB-IMPL::%DEFUN #<PACKAGE "PROTEST/BASE"> TE..
  1: (SB-FASL::LOAD-AS-FASL #<SB-SYS:FD-STREAM for "file /tmp/slimebA9Tm4.fasl" {1002F596A3}> NIL NIL)
  2: ((FLET SB-FASL::THUNK :IN LOAD))
  3: (SB-FASL::CALL-WITH-LOAD-BINDINGS #<CLOSURE (FLET SB-FASL::THUNK :IN LOAD) {7FFFF64FDBFB}> #<SB-SYS:FD-STREAM for "file /tmp/slimebA9Tm4.fasl" {1002F596A3}>)
  4: ((FLET SB-FASL::LOAD-STREAM :IN LOAD) #<SB-SYS:FD-STREAM for "file /tmp/slimebA9Tm4.fasl" {1002F596A3}> T)
  5: (LOAD #P"/tmp/slimebA9Tm4.fasl" :VERBOSE NIL :PRINT NIL :IF-DOES-NOT-EXIST T :EXTERNAL-FORMAT :DEFAULT)
  6: ((FLET SWANK/BACKEND:CALL-WITH-COMPILATION-HOOKS :IN "/home/phoe/.emacs.d/elpa/develop/slime-20171207.1712/swank/sbcl.lisp") #<CLOSURE (LAMBDA NIL :IN SWANK/BACKEND:SWANK-COMPILE-STRING) {1002F58C1B}>..
  7: ((FLET SWANK/BACKEND:SWANK-COMPILE-STRING :IN "/home/phoe/.emacs.d/elpa/develop/slime-20171207.1712/swank/sbcl.lisp") "(defun test () ..)
  8: ((LAMBDA NIL :IN SWANK:COMPILE-STRING-FOR-EMACS))
  9: ((LAMBDA NIL :IN SWANK::COLLECT-NOTES))
 --more--