dwarf::compilation_units(): avoid returning a null pointer
dwarf::compilation_units(): avoid returning a null pointer

aclements/libelfin,aclements/libelfin,tempbottle/libelfin,abenkhadra/libelfin,abenkhadra/libelfin,aclements/libelfin,aclements/libelfin,abenkhadra/libelfin,tempbottle/libelfin,abenkhadra/libelfin