Correctly check if child is flex by also accounting for undefined

Summary: We were incorrectly returning true for children which had no flex as the flex was nan and not 0.

Differential Revision: D4346712

fbshipit-source-id: 69ef0bb3fe5b4fcd3b3e2fe5aa348529be40252a

yoga/Yoga.c

@@ -825,7 +825,7 @@ static YGFlexDirection YGFlexDirectionCross(const YGFlexDirection flexDirection,
 
 static inline bool YGNodeIsFlex(const YGNodeRef node) {
   return (node->style.positionType == YGPositionTypeRelative &&
-          (node->style.flexGrow != 0 || node->style.flexShrink != 0 || node->style.flex != 0));
+          (YGNodeStyleGetFlexGrow(node) != 0 || YGNodeStyleGetFlexShrink(node) != 0));
 }
 
 static inline float YGNodeDimWithMargin(const YGNodeRef node, const YGFlexDirection axis) {