lib: test_objagg: Set error message in check_expect_hints_stats()

Smatch complains that the error message isn't set in the caller:

lib/test_objagg.c:923 test_hints_case2()
error: uninitialized symbol 'errmsg'.

This static checker warning only showed up after a recent refactoring
but the bug dates back to when the code was originally added. This
likely doesn't affect anything in real life.

Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Fixes: 0a020d416d0a ("lib: introduce initial implementation of object aggregation manager")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

lib/test_objagg.c

···
 	int err;
 
 	stats = objagg_hints_stats_get(objagg_hints);
-	if (IS_ERR(stats))
+	if (IS_ERR(stats)) {
+		*errmsg = "objagg_hints_stats_get() failed.";
 		return PTR_ERR(stats);
+	}
 	err = __check_expect_stats(stats, expect_stats, errmsg);
 	objagg_stats_put(stats);
 	return err;