Non-local exit from iterator has a false positive with #define_singleton_method

[AlexWayfer]

Hello!

I have a code:

str = 'foo'
str.define_singleton_method :bar do |baz|
  return unless baz
  replace baz
end

I think I can make PR for fixing this.

---

Expected behavior

No warnings from Rubocop.

Actual behavior

Warning about Non-local exit from iterator.

Steps to reproduce the problem

Check the file with this code (or similar) by Rubocop.

RuboCop version

$ rubocop -V
0.45.0 (using Parser 2.3.1.4, running on ruby 2.3.3 x86_64-linux)