Add TruffleBoundary to hasNext method of attrs iterator

(cherry picked from commit 8658bc4)

Author: steve-s

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/data/RAttributesLayout.java

@@ -286,6 +286,7 @@ static class Iter implements Iterator<RAttribute> {
         }
 
         @Override
+        @TruffleBoundary
         public boolean hasNext() {
             return iter.hasNext();
         }