Prevent crash when accessing child count, but child list is NULL.

Summary: Previously, we would preallocate Node's with a child list of 4. We recently removed that logic (see diff below), and as a result, if you tried to access a Node's list of children before it had been allocated, you would crash. I added a simple check to protect from crashes, the operation of the check is O(1) so we shouldn't see a perf hit.

Reviewed By: emilsjolander

Differential Revision: D4104093

fbshipit-source-id: cd7b09818759aa76415b97e241f1a6746a2bc50c

tests/CSSLayoutDefaultValuesTest.cpp

@@ -13,6 +13,9 @@
 TEST(CSSLayoutTest, assert_default_values) {
   const CSSNodeRef root = CSSNodeNew();
 
+  ASSERT_EQ(0, CSSNodeChildCount(root));
+  ASSERT_EQ(NULL, CSSNodeGetChild(root, 1));
+
   ASSERT_EQ(CSSDirectionInherit, CSSNodeStyleGetDirection(root));
   ASSERT_EQ(CSSFlexDirectionColumn, CSSNodeStyleGetFlexDirection(root));
   ASSERT_EQ(CSSJustifyFlexStart, CSSNodeStyleGetJustifyContent(root));