[ASTextNode2] Check that rangeOut is non-nil before dereferencing (#1980)

rangeOut is a nullable parameter, but we are treating it as though it is always not null. Make sure it is not null before dereferencing it.

Author: rcancro

Source/ASTextNode2.mm

@@ -702,7 +702,9 @@ - (id)_linkAttributeValueAtPoint:(CGPoint)point
         continue;
       }
 
-      *rangeOut = NSIntersectionRange(visibleRange, effectiveRange);
+      if (rangeOut != NULL) {
+        *rangeOut = NSIntersectionRange(visibleRange, effectiveRange);
+      }
 
       if (attributeNameOut != NULL) {
         *attributeNameOut = attributeName;