odb: clear backend errors on successful read

We go through the different backends in order, so it's not an error if
at least one of the backends has the data we want.

src/odb.c

@@ -783,6 +783,7 @@ int git_odb_read(git_odb_object **out, git_odb *db, const git_oid *id)
 		return error;
 	}
 
+	giterr_clear();
 	if ((object = odb_object__alloc(id, &raw)) == NULL)
 		return -1;