Missing null check of rpcServer.scheduler.executor causes NPE with invalid value of hbase.client.default.rpc.codec

When `hbase.client.default.rpc.codec` is set to `org.apache.hadoop.hbase.regionserver.wal.SecureWALCellCodec` the method `createRpcClientRTEDuringConnectionSetup` gives a NoSuchMethodException for the provided default codec value. 
 
Since there was an exception, the code inside try block is not executed which contained the below code line 

try (AbstractRpcClient<?> client = createRpcClientRTEDuringConnectionSetup(clientConf))
{       rpcServer.start();       . . .     }

When finally block is executed which contains 

finally
{       rpcServer.stop();     }

The method `rpcServer.stop()` eventually makes call to `executor.shutdown()`. A null pointer exception is returned as  `rpcServer.scheduler.executor`  was never instantiated, which happens inside `rpcServer.start()`
 

java.lang.NullPointerException
at org.apache.hadoop.hbase.ipc.FifoRpcScheduler.stop(FifoRpcScheduler.java:71)
at org.apache.hadoop.hbase.ipc.SimpleRpcServer.stop(SimpleRpcServer.java:441)
at org.apache.hadoop.hbase.ipc.AbstractTestIPC.testRTEDuringConnectionSetup(AbstractTestIPC.java:194)