hash: add asserts to help detect bad usage

For example trying to add entries after the hash has been "cleaned up"

Closes #8115
diff --git a/lib/hash.c b/lib/hash.c
index 12e7aa5..f04c462 100644
--- a/lib/hash.c
+++ b/lib/hash.c
@@ -110,7 +110,9 @@
 {
   struct Curl_hash_element  *he;
   struct Curl_llist_element *le;
-  struct Curl_llist *l = FETCH_LIST(h, key, key_len);
+  struct Curl_llist *l;
+  DEBUGASSERT(h->slots);
+  l = FETCH_LIST(h, key, key_len);
 
   for(le = l->head; le; le = le->next) {
     he = (struct Curl_hash_element *) le->ptr;
@@ -139,7 +141,9 @@
 int Curl_hash_delete(struct Curl_hash *h, void *key, size_t key_len)
 {
   struct Curl_llist_element *le;
-  struct Curl_llist *l = FETCH_LIST(h, key, key_len);
+  struct Curl_llist *l;
+  DEBUGASSERT(h->slots);
+  l = FETCH_LIST(h, key, key_len);
 
   for(le = l->head; le; le = le->next) {
     struct Curl_hash_element *he = le->ptr;
@@ -163,6 +167,7 @@
   struct Curl_llist *l;
 
   if(h) {
+    DEBUGASSERT(h->slots);
     l = FETCH_LIST(h, key, key_len);
     for(le = l->head; le; le = le->next) {
       struct Curl_hash_element *he = le->ptr;