Fix NPE when closing cloned JdbcConnection

[ThePrez]

closing a cloned JdbcConnection (by calling the copy constructor) results in an NPE:

org.h2.jdbc.JdbcSQLNonTransientException: General error: "java.lang.NullPointerException" [50000-210]
	at org.h2.message.DbException.getJdbcSQLException(DbException.java:573)
	at org.h2.message.DbException.getJdbcSQLException(DbException.java:496)
	at org.h2.message.DbException.get(DbException.java:216)
	at org.h2.message.DbException.convert(DbException.java:414)
	at org.h2.message.DbException.toSQLException(DbException.java:386)
	at org.h2.message.TraceObject.logAndConvert(TraceObject.java:365)
	at org.h2.jdbc.JdbcConnection.close(JdbcConnection.java:368)
Caused by: java.lang.NullPointerException
	at org.h2.util.CloseWatcher.unregister(CloseWatcher.java:100)
	at org.h2.jdbc.JdbcConnection.close(JdbcConnection.java:334)
	... 2 more

This is because the clone nulls out the watcher field of the cloned object, which I believe is by design:

h2database/h2/src/main/org/h2/jdbc/JdbcConnection.java

Line 154 in 465399f

this.watcher = null;

Circumvented with a simple null check which I propose as a small fix.

[grandinj]

do you have a test case for this?

[ThePrez]

I can certainly make one. I'll put this in draft status until I add a test to this PR.