cmucl: fix error handling in %LOAD-FOREIGN-LIBRARY
cmucl: fix error handling in %LOAD-FOREIGN-LIBRARY

Patch courtesy of Matthias Benkard.

akovalenko/cffi,kingcheez/cffi-cheez,3b/cffi,cffi/cffi,stassats/cffi,dkochmanski/cffi,3b/cffi,dochang/cffi,marsijanin/cffi,3b/cffi,dochang/cffi,attila-lendvai/cffi,dkochmanski/cffi,wesen/cffi,akovalenko/cffi,mcna/cffi,stassats/cffi,kingcheez/cffi-cheez,mcna/cffi,marsijanin/cffi,cffi/cffi,wesen/cffi,dkochmanski/cffi,akovalenko/cffi,stassats/cffi,kingcheez/cffi-cheez,cffi/cffi,wesen/cffi,marsijanin/cffi,attila-lendvai/cffi,dochang/cffi