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 <lkp@intel.com>
Closes: https://lore.kernel.org/r/202506281403.DsuyHFTZ-lkp@intel.com/
Fixes: 0a020d4 ("lib: introduce initial implementation of object aggregation manager")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/8548f423-2e3b-4bb7-b816-5041de2762aa@sabinyo.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Dan Carpenter

lib/test_objagg.c

@@ -899,8 +899,10 @@ static int check_expect_hints_stats(struct objagg_hints *objagg_hints,
 	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;