Fix crash min-max refactoring checker

[cdce8p]

Steps

• Add yourself to CONTRIBUTORS if you are a new contributor.
• Add a ChangeLog entry describing what your PR does.
• If it's a new feature or an important bug fix, add a What's New entry in
  doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.

Description

Fix crash for

var = 1
if var == -1:
    var = None

Since this checker hasn't been released yet, I don't think it's necessary to write a changelog entry.

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes: #4379

/CC: @Pierre-Sassoulas, @Qwiddle13, @manderj

[coveralls]

Coverage increased (+0.001%) to 91.623% when pulling 3554185 on cdce8p:fix-min-max-checker into e826cec on PyCQA:master.

[Pierre-Sassoulas]

Good job catching that !