Fix issues raised on pull request

This resolves the comments on pull request #590

src/errors.c

@@ -124,7 +124,7 @@ void giterr_set(int error_class, const char *string, ...)
 	va_list arglist;
 	git_error *error;
 	const char *oserr =
-		(error_class == GITERR_OS && errno > 0) ? strerror(errno) : NULL;
+		(error_class == GITERR_OS && errno != 0) ? strerror(errno) : NULL;
 
 	error = &GIT_GLOBAL->error_t;
 	free(error->message);

src/odb.c

@@ -319,12 +319,8 @@ static int add_backend_internal(git_odb *odb, git_odb_backend *backend, int prio
 
 	assert(odb && backend);
 
-	if (backend->odb != NULL && backend->odb != odb) {
-		/*
-		 * TODO: Not sure how to convert this!
-		 */
-		return git__throw(GIT_EBUSY, "The backend is already owned by another ODB");
-	}
+	/* Check if the backend is already owned by another ODB */
+	assert(!backend->odb || backend->odb == odb);
 
 	internal = git__malloc(sizeof(backend_internal));
 	GITERR_CHECK_ALLOC(internal);