fix bug of NaN left style

[enbochen]

This PR solves the bug after upgrading to React Native 0.58.0

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[sharplet]

Beside the typo, this fixed the issue I was seeing after upgrading to React Native 0.58.0 where Navigator transitions were completely broken. Thanks!

[sharplet]

FWIW, these transitions still appear to work on 0.59.0, but not on the latest version, 0.59.8 (instead of an animation, the new screen just pops in).

[SpencerLynn]

This likely will fix #37

This fix is probably good, but should the styles also be fixed to not directly lead to this issue?