ZOOKEEPER-5051: Avoid NPE closing 4lw connection during startup

[PAC-MAJ]

Problem

When a 4-letter command is sent while the ZooKeeper server is starting,
the connection close path can call ZooKeeperServer.removeCnxn() before
startdata() has initialized zkDb.

This can trigger:

java.lang.NullPointerException: Cannot invoke
"org.apache.zookeeper.server.ZKDatabase.removeCnxn(...)"
because "this.zkDb" is null

But this also hangs any client waiting for connection close.

Fix

Guard ZooKeeperServer.removeCnxn() so it only delegates to zkDb
when zkDb has already been initialized.

Test

Added ZooKeeperServerTest#testRemoveCnxnBeforeStartData.
The test creates a ZooKeeperServer but intentionally does not call
startdata(), then verifies that removeCnxn() does not throw.

[PAC-MAJ]

The latest failure appears to be a Jenkins infrastructure/agent issue.
The build failed during the post-stage JUnit result collection with hudson.remoting.Channel$OrderlyShutdown / RequestAbortedException on agent hadoop16, not during Maven test execution ( I see BUILD SUCCESS ).

Could someone kindly rerun the precommit job?