child-create: Fix crash when retrying CHILD_SA rekeying due to a DH group mismatch

If the responder declines our KE payload during a CHILD_SA rekeying migrate()
is called to reuse the child-create task.  But the child-rekey task then
calls the same method again.

Fixes: 32df0d81fb ("child-create: Destroy nonceg in migrate()")

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

@@ -1596,6 +1596,7 @@ METHOD(task_t, migrate, void,
 	this->tsi = NULL;
 	this->tsr = NULL;
 	this->dh = NULL;
+	this->nonceg = NULL;
 	this->child_sa = NULL;
 	this->mode = MODE_TUNNEL;
 	this->ipcomp = IPCOMP_NONE;