hsts: remove assert for zero length domain

A zero length domain can happen if the HSTS parser is given invalid
input data which is not unheard of and is done by the fuzzer.

Follow-up from cfe7902

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65661

Closes #12676

lib/hsts.c

@@ -117,8 +117,6 @@ static CURLcode hsts_create(struct hsts *h,
  bool subdomains,
  curl_off_t expires)
 {
- struct stsentry *sts;
- char *duphost;
  size_t hlen;
  DEBUGASSERT(h);
  DEBUGASSERT(hostname);
@@ -127,25 +125,23 @@ static CURLcode hsts_create(struct hsts *h,
  if(hlen && (hostname[hlen - 1] == '.'))
  /* strip off any trailing dot */
  --hlen;
- DEBUGASSERT(hlen);
- if(!hlen)
- /* no host name left */
- return CURLE_BAD_FUNCTION_ARGUMENT;
-
- sts = hsts_entry();
- if(!sts)
- return CURLE_OUT_OF_MEMORY;
+ if(hlen) {
+ char *duphost;
+ struct stsentry *sts = hsts_entry();
+ if(!sts)
+ return CURLE_OUT_OF_MEMORY;
+
+ duphost = Curl_memdup0(hostname, hlen);
+ if(!duphost) {
+ free(sts);
+ return CURLE_OUT_OF_MEMORY;
+ }
 
- duphost = Curl_memdup0(hostname, hlen);
- if(!duphost) {
- free(sts);
- return CURLE_OUT_OF_MEMORY;
+  sts->host = duphost;
+  sts->expires = expires;
+ sts->includeSubDomains = subdomains;
+ Curl_llist_insert_next(&h->list, h->list.tail, sts, &sts->node);
  }
-
- sts->host = duphost;
- sts->expires = expires;
- sts->includeSubDomains = subdomains;
- Curl_llist_insert_next(&h->list, h->list.tail, sts, &sts->node);
  return CURLE_OK;
 }