runtime error: member access within null pointer of type 'ListNode，但是加上else就能成功运行了

while(l1!=NULL&&l2!=NULL){
if(l1->val>=l2->val){
cur->next=l2;
cur=cur->next;
l2=l2->next;
}
if(l1->val<l2->val){
cur->next=l1;
cur=cur->next;
l1=l1->next;
}
}