Fix that Hash may not contain any empty buckets

The Hash implementation assumed that there were always empty buckets, but
sometimes there were only active or deleted buckets (no empty buckets).
Therefore, fix it so that this situation does not occur.

### Example

```ruby
# example.rb
class A
  attr_reader :v
  def initialize(v) @v = v end
  def ==(o) @v == o.v end
  def hash; @v end
  def to_s; "#{self.class}[#{@v}]" end
  alias eql? ==
  alias inspect to_s
end

keys = (0..31).map{A.new(_1)}
h = {}
(0..16).each{h[keys[_1]] = _1}
(17..31).each do
  k = keys[_1]
  h[k] = _1
  h.delete(k)
end
p h.keys
```

#### Before this patch:

```console
$ bin/mruby example.rb
[A[0], A[1], A[2], A[3], A[4], A[5], A[6], A[7], A[8], A[9], A[10], A[11], A[12], A[13], A[14], A[15], A[16], A[30], A[31]]
```

#### After this patch:

```console
$ bin/mruby example.rb
[A[0], A[1], A[2], A[3], A[4], A[5], A[6], A[7], A[8], A[9], A[10], A[11], A[12], A[13], A[14], A[15], A[16]]
```

src/hash.c

@@ -52,10 +52,10 @@
  *   the number of used slots and the number of empty slots.
  */
 
+#define EA_N_RESERVED_INDICES 2  /* empty and deleted */
 #define EA_INCREASE_RATIO 6 / 5 + 6
 #define EA_MAX_INCREASE UINT16_MAX
-#define EA_MAX_CAPA  /* `- 2` means reserved indices (empty and deleted) */   \
-  U32(lesser(IB_MAX_CAPA - 2, MRB_INT_MAX))
+#define EA_MAX_CAPA U32(lesser(IB_MAX_CAPA - EA_N_RESERVED_INDICES, MRB_INT_MAX))
 #define IB_MAX_CAPA (U32(1) << IB_MAX_BIT)
 #define IB_TYPE_BIT 32
 #define IB_INIT_BIT (                                                         \
@@ -360,7 +360,7 @@ ea_next_capa_for(uint32_t size, uint32_t max_capa)
   }
   else {
     /*
-     * For 32-bit CPU, the theoretical value of max EA capa is
+     * For 32-bit CPU, the theoretical value of maximum EA capacity is
      * `UINT32_MAX / sizeof (hash_entry)`. At this time, if
      * `EA_INCREASE_RATIO` is the current value, 32-bit range will not be
      * exceeded during the calculation of `capa`, so `size_t` is used.
@@ -839,14 +839,18 @@ ht_set(mrb_state *mrb, struct RHash *h, mrb_value key, mrb_value val)
     if (size != ht_ea_n_used(h)) ea_compress(ht_ea(h), ht_ea_n_used(h));
     ht_init(mrb, h, size, ht_ea(h), ht_ea_capa(h), h_ht(h), ++ib_bit_width);
   }
-  else if (ht_ea_capa(h) == ht_ea_n_used(h) && size != ht_ea_n_used(h)) {
+  else if (size != ht_ea_n_used(h)) {
+    if (ib_capa - EA_N_RESERVED_INDICES <= ht_ea_n_used(h)) goto compress;
+    if (ht_ea_capa(h) == ht_ea_n_used(h)) {
       if (size <= AR_MAX_SIZE) {ht_set_as_ar(mrb, h, key, val); return;}
       if (ea_next_capa_for(size, EA_MAX_CAPA) <= ht_ea_capa(h)) {
+       compress:
         ea_compress(ht_ea(h), ht_ea_n_used(h));
         ht_adjust_ea(mrb, h, size, ht_ea_capa(h));
         ht_init(mrb, h, size, ht_ea(h), ht_ea_capa(h), h_ht(h), ib_bit_width);
       }
     }
+  }
   ht_set_without_ib_adjustment(mrb, h, key, val);
 }