Fix possible temporary cache entry corruption

If for some reason the cache queue lock couldn't be locked, the created
entry would remain in the hash table.  This could potentially lead to
two threads obtaining a reference to this temporary entry, and calling
cache_entry_unref() on it, leading to double frees and other nasty
problems.

It's very unlikely that this condition will happen, as
pthread_rwlock_wrlock() should fail only when a dead lock would occur.

Reported by immerse

common/lwan-cache.c

@@ -239,6 +239,11 @@ struct cache_entry *cache_get_and_ref_entry(struct cache *cache,
             pthread_rwlock_unlock(&cache->queue.lock);
         } else {
             convert_to_temporary(entry);
+
+            /* Ensure item is removed from the hash table; otherwise,
+             * another thread could potentially get another reference
+             * to this entry and cause an invalid memory access. */
+            hash_del(cache->hash.table, entry->key);
         }
     } else {
         /* Either there's another item with the same key (-EEXIST), or