child-create: Don't reset DH group when retrying after INVALID_KE_PAYLOAD

migrate() is called before retrying.

Fixes: 0184a69b7b ("child-create: Properly handle DH group during
migration when reestablishing")

src/libcharon/sa/ikev2/tasks/child_create.c

@@ -1853,7 +1853,7 @@ METHOD(task_t, migrate, void,
 	{
 		this->proposals->destroy_offset(this->proposals, offsetof(proposal_t, destroy));
 	}
-	if (!this->rekey)
+	if (!this->rekey && !this->retry)
 	{
 		this->dh_group = MODP_NONE;
 	}