Fixed a problem with RScalar leaking from vector update.

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/access/vector/CachedReplaceVectorNode.java

@@ -215,7 +215,7 @@ final class CachedReplaceVectorNode extends CachedVectorNode {
         int replacementLength = positionsCheckNode.getSelectedPositionsCount(positionProfiles);
         if (emptyReplacementProfile.profile(replacementLength == 0)) {
             /* Nothing to modify */
-            return vector;
+            return vector.materialize();
         }
 
         if (valueLengthOneProfile.profile(valueLength != 1)) {