Compilation of (describe 'foo)

This is related to #24 (closed), but we replaced (compile 'foo) with (describe 'foo). Compiling works fine, but loading the resulting fasl produces

Type-error in KERNEL::OBJECT-NOT-TYPE-ERROR-HANDLER:
   NIL is not of type C::DEBUG-SOURCE
   [Condition of type TYPE-ERROR]

Restarts:
  0: [CONTINUE] Return NIL from load of "bug-24".
  1: [RETRY   ] Retry SLIME REPL evaluation request.
  2: [ABORT   ] Return to SLIME's top level.
  3:            Return to Top-Level.

Debug  (type H for help)

(LISP::PRINT-COMPILED-FROM #<Code Object "DEFUN FOO" {483AEF67}>)

This is because (c::debug-info-source info) returns NIL.