Applied patch submitted by Mats Lofkvist fixing serious threading problem introduced...

Applied patch submitted by Mats Lofkvist fixing serious threading problem introduced in beta3.
Fixed bug with using setNull()(or setXXX(x, null))  and serverside prepare statements.
Improved error message when using a connection object that has already been closed.

 Modified Files:
 	jdbc/org/postgresql/errors.properties
 	jdbc/org/postgresql/core/Encoding.java
 	jdbc/org/postgresql/core/QueryExecutor.java
 	jdbc/org/postgresql/jdbc1/AbstractJdbc1Connection.java
 	jdbc/org/postgresql/jdbc1/AbstractJdbc1Statement.java