Fix crash if #inspect does not return a string value

case 1
~~~
class A; def inspect; 1; end; end
A.new.a
~~~

case 2

~~~
class A
  def self.inspect
    1
  end
  alias_method :a, :b
end
~~~
Merge pull request #2668 from sdottaka/fix-crash-for-inspect

Fix crash if #inspect does not return a string value