Don't use ensure to fix crash of mruby-eval test.

test/assert.rb

@@ -63,10 +63,9 @@ def assert(str = 'Assertion failed', iso = '')
       $asserts.push(assertion_string('Error: ', str, iso, e))
       $kill_test += 1
       t_print('X')
+    end
   end
-  ensure
-    $mrbtest_assert = nil
-  end
+  $mrbtest_assert = nil
   t_print("\n") if $mrbtest_verbose
 end