Fix heap-buffer-overflow for small Hash (HT) in Hash#rehash

### Example

##### example.rb

```ruby
h = {}
(1..17).each{h[_1] = _1}
(1..16).each{h.delete(_1)}
h.rehash
```

##### ASAN report

```console
$ bin/mruby example.rb
==52587==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000006998 at pc 0x55a29cddf96b bp 0x7fff7b1b1720 sp 0x7fff7b1b1710
READ of size 4 at 0x602000006998 thread T0
    #0 0x55a29cddf96a in ib_it_next /mruby/src/hash.c:639
    #1 0x55a29cde2ca2 in ht_rehash /mruby/src/hash.c:900
    #2 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #3 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #4 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #5 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #6 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #7 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #8 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #9 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #10 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #11 0x55a29cdb4a6d in _start (/mruby/bin/mruby+0x2a3a6d)

0x602000006998 is located 0 bytes to the right of 8-byte region [0x602000006990,0x602000006998)
allocated by thread T0 here:
    #0 0x7f24f01cfffe in __interceptor_realloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dffe)
    #1 0x55a29ceb9440 in mrb_default_allocf /mruby/src/state.c:68
    #2 0x55a29cdba747 in mrb_realloc_simple /mruby/src/gc.c:228
    #3 0x55a29cdba928 in mrb_realloc /mruby/src/gc.c:242
    #4 0x55a29cde12e5 in ht_init /mruby/src/hash.c:749
    #5 0x55a29cde2b8e in ht_rehash /mruby/src/hash.c:897
    #6 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #7 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #8 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #9 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #10 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #11 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #12 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #13 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #14 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
```

src/hash.c

@@ -718,6 +718,7 @@ ib_bit_for(uint32_t size)
 static uint32_t
 ib_byte_size_for(uint32_t ib_bit)
 {
+  mrb_assert(IB_INIT_BIT <= ib_bit);
   uint32_t ary_size = IB_INIT_BIT == 4 ?
     ib_bit_to_capa(ib_bit) * 2 / IB_TYPE_BIT * ib_bit / 2 :
     ib_bit_to_capa(ib_bit) / IB_TYPE_BIT * ib_bit;
@@ -892,7 +893,13 @@ static void
 ht_rehash(mrb_state *mrb, struct RHash *h)
 {
   /* see comments in `h_rehash` */
-  uint32_t size = ht_size(h), w_size = 0, ea_capa = ht_ea_capa(h);
+  uint32_t size = ht_size(h);
+  if (size <= AR_MAX_SIZE) {
+    ht_to_ar(mrb, h);
+    ar_rehash(mrb, h);
+    return;
+  }
+  uint32_t w_size = 0, ea_capa = ht_ea_capa(h);
   hash_entry *ea = ht_ea(h);
   ht_init(mrb, h, 0, ea, ea_capa, h_ht(h), ib_bit_for(size));
   ht_set_size(h, size);

test/t/hash.rb

@@ -944,6 +944,14 @@ assert('Hash#rehash') do
   h = {}
   assert_same(h, h.rehash)
   assert_predicate(h, :empty?)
+
+  h = {}
+  (1..17).each{h[_1] = _1 * 2}
+  (2..16).each{h.delete(_1)}
+  assert_same(h, h.rehash)
+  assert_equal([[1, 2], [17, 34]], h.to_a)
+  assert_equal(2, h.size)
+  [1, 17].each{assert_equal(_1 * 2, h[_1])}
 end
 
 assert('#eql? receiver should be specified key') do