add null check for keyboardFrame variable in KeyboardAvoidingView component

Summary:
This is a critical change and should be pushed to stabel asap

The reason for the pull request is to prevent crashes that can occure if the function relativeKeyboardHeight(keyboardFrame: ScreenRect) got null as an input variable which sometimes does happen.
Closes https://github.com/facebook/react-native/pull/10287

Differential Revision: D3988359

Pulled By: bestander

fbshipit-source-id: 0d1052b590b2684907ea6f7d6b4fe9b89989d4dd

Libraries/Components/Keyboard/KeyboardAvoidingView.js

@@ -87,7 +87,7 @@ const KeyboardAvoidingView = React.createClass({
 
   relativeKeyboardHeight(keyboardFrame: ScreenRect): number {
     const frame = this.frame;
-    if (!frame) {
+    if (!frame || !keyboardFrame) {
       return 0;
     }