Fixed reference_p() to return false for NULL.

This indirectly fixes one of the issues exposed in the following:

https://github.com/sparklemotion/nokogiri/issues/1047
https://github.com/rubinius/rubinius/issues/2844

This is not the root cause of the Nokogiri-relate segvs, which
appear to be Nokogiri (libxml2) releasing data for a node while
a Data object wrapping it is still reachable and hence is having
mark() called on it.