Fix ensure with yield context on break and return

How to reproduce:

    class A
      def x
        yield
      ensure
        y
      end

      def y
      end
    end

    # Work
    A.new.x do
    end

    # Not work
    # trace:
    # 	[2] /tmp/a.rb:5:in A.x
    # 	[0] /tmp/a.rb:15
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    A.new.x do
      break
    end

    # trace:
    # 	[2] /tmp/a.rb:5:in A.call
    # 	[0] /tmp/a.rb:19
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    lambda do
      A.new.x do
        return
      end
    end.call

`self` in ensure is broken when yield and break/return are used.

src/vm.c

@@ -1514,6 +1514,7 @@ RETRY_TRY_BLOCK:
               localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
               goto L_RAISE;
             }
+            mrb->c->stack = mrb->c->ci->stackent;
             mrb->c->ci = ci;
             break;
           }
@@ -1548,6 +1549,7 @@ RETRY_TRY_BLOCK:
             c->prev = NULL;
           }
           ci = mrb->c->ci;
+          mrb->c->stack = ci->stackent;
           mrb->c->ci = mrb->c->cibase + proc->env->cioff + 1;
           while (ci > mrb->c->ci) {
             if (ci[-1].acc == CI_ACC_SKIP) {

test/t/ensure.rb

+##
+# ensure Test
+
+assert('ensure - context - yield') do
+  class EnsureYieldBreak
+    attr_reader :ensure_context
+    def try
+      yield
+    ensure
+      @ensure_context = self
+    end
+  end
+
+  yielder = EnsureYieldBreak.new
+  yielder.try do
+  end
+  assert_equal yielder, yielder.ensure_context
+end
+
+assert('ensure - context - yield and break') do
+  class EnsureYieldBreak
+    attr_reader :ensure_context
+    def try
+      yield
+    ensure
+      @ensure_context = self
+    end
+  end
+
+  yielder = EnsureYieldBreak.new
+  yielder.try do
+    break
+  end
+  assert_equal yielder, yielder.ensure_context
+end
+
+assert('ensure - context - yield and return') do
+  class EnsureYieldBreak
+    attr_reader :ensure_context
+    def try
+      yield
+    ensure
+      @ensure_context = self
+    end
+  end
+
+  yielder = EnsureYieldBreak.new
+  lambda do
+    yielder.try do
+      return
+    end
+  end.call
+  assert_equal yielder, yielder.ensure_context
+end