SEGV when lookup_* returns NULL

hoi :)

When trying to an unmodified GIT on a repository with submodules
it segfaults a lot.

All the lookup_{blob,tree,commit} functions check that the object
really is of the requested type and return NULL otherwise.
However this NULL pointer is not checked in the calling functions.

Should we make lookup_* to just die when invoked on another object-type?
Or modify all the callers?
Is there a sane error-handling strategy besides dying in this case?
Really checking all the return values in the whole chain would be
a lot of work.

-- 
Martin Waitz