adding extra checking in _clearListener method

[AVStarikovich]

When I using google map component with marker component multiple times on different pages, it makes error like this:

So, it solves a trouble.

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[AVStarikovich]

I signed it!

[googlebot]

CLAs look good, thanks!

[FluorescentHallucinogen]

@e111077 @ebidel PTAL.