Add xassert() to on_http_connection_finish()

Catch when con->ref and con->con are not pointing to the same conmgr
connection.

Issue: 50189
diff --git a/src/slurmrestd/http.c b/src/slurmrestd/http.c
index 383c955..6748ee4 100644
--- a/src/slurmrestd/http.c
+++ b/src/slurmrestd/http.c
@@ -803,6 +803,7 @@
 	xassert(!context->auth);
 	FREE_NULL_REST_AUTH(context->auth);
 
+	xassert(conmgr_fd_get_ref(context->ref) == context->con);
 	conmgr_fd_free_ref(&context->ref);
 	context->con = NULL;