A fix-up of refence counting when setting an attribute

...ary/src/com/oracle/truffle/r/library/stats/RMultinom.java

...tin/src/com/oracle/truffle/r/nodes/builtin/base/Attr.java

...cle/truffle/r/nodes/access/vector/ExtractListElement.java

...truffle/r/nodes/access/vector/WriteIndexedVectorNode.java

...m/oracle/truffle/r/nodes/attributes/SetAttributeNode.java

...cle/truffle/r/nodes/attributes/SetFixedAttributeNode.java

...ruffle/r/nodes/attributes/SpecialAttributesFunctions.java

...fle/r/nodes/attributes/UpdateShareableChildValueNode.java

...racle/truffle/r/test/builtins/TestBuiltin_attrassign.java

.../com/oracle/truffle/r/test/builtins/TestBuiltin_list.java