Fix warnings of casting and null pointer handling

Summary: There were warnings of castings and null pointer handling in yoga.cpp. This diff fixes the warnings. The issue was brought up here https://github.com/facebook/react-native/issues/17274

Reviewed By: emilsjolander

Differential Revision: D6675111

fbshipit-source-id: 884659fabb05033b4d43d3aa6629e22481d39b7e

yoga/YGNode.cpp

@@ -385,7 +385,7 @@ void YGNode::cloneChildrenIfNeeded() {
   // YGNodeRemoveChild in yoga.cpp has a forked variant of this algorithm
   // optimized for deletions.
 
-  const uint32_t childCount = children_.size();
+  const uint32_t childCount = static_cast<uint32_t>(children_.size());
   if (childCount == 0) {
     // This is an empty set. Nothing to clone.
     return;