Some systems failed to build for Quicklisp dist

[quicklisp]

Building with SBCL 2.1.6 / ASDF 3.3.1 for quicklisp dist creation.

Trying to build commit id 6da7fe1

cl-bunny.examples fails to build because of a failure in cl-bunny.

cl-bunny.test fails to build because of a failure in cl-bunny.

cl-bunny fails to build with the following error:

; caught WARNING:
;   Constant NIL conflicts with its asserted type CL-BUNNY::CHANNEL.
;   See also:
;     The SBCL Manual, Node "Handling of Types"
...
Unhandled UIOP/LISP-BUILD:COMPILE-FILE-ERROR in thread #<SB-THREAD:THREAD "main thread" RUNNING {1040E10843}>: COMPILE-FILE-ERROR while compiling #<CL-SOURCE-FILE "cl-bunny" "src" "exchange">

Full log here

[brown]

This pull request fixes the problem: #10