Hashtable: Always choose next prime when growing "buckets"

The next prime number for the size of "buckets" may be less than 2 times
the old size. That would cause a skip in the prime number selection,
effectively making the new "buckets" size nearly 4 times the old size
(in constrast with 2 times the old size).

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09

Hashtable.c

@@ -230,7 +230,7 @@ void Hashtable_put(Hashtable* this, ht_key_t key, void* value) {
       CRT_fatalError("Hashtable: size overflow");
 
    if (this->items >= this->size * 7 / 10)
-      Hashtable_setSize(this, 2 * this->size);
+      Hashtable_setSize(this, this->size + 1);
 
    insert(this, key, value);