Raise an exception if ar_table is converted to st_table during iteration

ar_table may be converted to st_table by `ar_force_convert_table`.
If the conversion occurs during the iteration of ar_table, the iteration
may lead to memory corruption.

This change prevents the catastrophy by throwing an exception when the
conversion is detected.

This issue is reported by [SuperS](https://hackerone.com/superss)

hash.c

@@ -811,6 +811,14 @@ ar_add_direct_with_hash(VALUE hash, st_data_t key, st_data_t val, st_hash_t hash
     }
 }
 
+static void
+ensure_ar_table(VALUE hash)
+{
+    if (!RHASH_AR_TABLE_P(hash)) {
+        rb_raise(rb_eRuntimeError, "hash representation was changed during iteration");
+    }
+}
+
 static int
 ar_general_foreach(VALUE hash, st_foreach_check_callback_func *func, st_update_callback_func *replace, st_data_t arg)
 {
@@ -822,6 +830,7 @@ ar_general_foreach(VALUE hash, st_foreach_check_callback_func *func, st_update_c
 
             ar_table_pair *pair = RHASH_AR_TABLE_REF(hash, i);
             enum st_retval retval = (*func)(pair->key, pair->val, arg, 0);
+            ensure_ar_table(hash);
             /* pair may be not valid here because of theap */
 
             switch (retval) {
@@ -896,6 +905,7 @@ ar_foreach_check(VALUE hash, st_foreach_check_callback_func *func, st_data_t arg
             hint = ar_hint(hash, i);
 
             retval = (*func)(key, pair->val, arg, 0);
+            ensure_ar_table(hash);
             hash_verify(hash);
 
             switch (retval) {
@@ -956,6 +966,7 @@ ar_update(VALUE hash, st_data_t key,
     old_key = key;
     retval = (*func)(&key, &value, arg, existing);
     /* pair can be invalid here because of theap */
+    ensure_ar_table(hash);
 
     switch (retval) {
       case ST_CONTINUE: