Fix potentially use of wrong method cache

#### Example (with `MRB_METHOD_CACHE`)

  ```ruby
  GC.start
  c = Class.new
  p c            #=> #<Class:0x7fd6a180e790>
  c.new          #=> cache `c.new`
  c = nil
  GC.start       #=> `c` is GCed
  r = Range.dup
  p r            #=> #<Class:0x7fd6a180e790>
                 #   [same pointer as `c`]
  r.new(2, 3)    #=> ArgumentError: 'initialize':
                 #   wrong number of arguments (2 for 0)
                 #   [`c.new` is called instead of `r.new`]
  ```

#### Cause

  An entry of method cache is identified by class pointer and method
  id. However, reusing memory after GC may create a class with the same
  pointer as the cached class.

#### Treatment

  Cleared method caches of the class when the class is GCed.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/gc.c b/src/gc.c
index 40c85a373..8d9a1806c 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -807,10 +807,12 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
   case MRB_TT_SCLASS:
     mrb_gc_free_mt(mrb, (struct RClass*)obj);
     mrb_gc_free_iv(mrb, (struct RObject*)obj);
+    mrb_mc_clear_by_class(mrb, (struct RClass*)obj);
     break;
   case MRB_TT_ICLASS:
     if (MRB_FLAG_TEST(obj, MRB_FL_CLASS_IS_ORIGIN))
       mrb_gc_free_mt(mrb, (struct RClass*)obj);
+    mrb_mc_clear_by_class(mrb, (struct RClass*)obj);
     break;
   case MRB_TT_ENV:
     {