avro

AVRO-742. Maintain reference count for singleton objects.

We use a singleton struct for instances of the null value type, and for
each of the primitive schema types.  This means that we need to increase
the reference count of each singleton each time the appropriate
constructor is called; otherwise, the reference count will get wonky
when we call avro_whatever_decref().  That won't cause any memory
corruption, since we never free any singleton objects, regardless of
what their reference counts do, but still.

git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1062747 13f79535-47bb-0310-9956-ffa450edef68