Fix error on Clozure CL

Hi,

The trivial-shell:shell-command didn't work on ccl.

? (trivial-shell:shell-command "ls")
> Error: The value NIL is not of the expected type ARRAY.
> While executing: CCL::ARRAY-DATA-AND-OFFSET, in process listener(1).
> Type :POP to abort, :R for a list of available restarts.
> Type :? for other options.
1 > 

So I've fix it.
Would you consider pulling the branch fix-error-on-ccl on ktateish/trivial-shell .