Fix astroid.Inference error for undefined-variables with len()

[cdce8p]

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Description

Wrap node.infer() in try ... except to catch astroid.InferenceError if variable is undefined.
The error seems to be a result of e50d733

--

This could be included in 2.7.3

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #4215

[coveralls]

Coverage increased (+0.002%) to 91.429% when pulling 3c09370 on cdce8p:fix-len-as-condition_InferenceError into ba4941b on PyCQA:master.

[Pierre-Sassoulas]

Thank you for the quick fix.