NPE was thrown when I set server name prefix for debugd as following:

```
$ jhsdb -J-Dsun.jvm.hotspot.rmi.serverNamePrefix=test debugd --pid 781
Attaching to process ID 781 and starting RMI services, please wait...
Error attaching to process or starting server: sun.jvm.hotspot.debugger.DebuggerException: java.lang.NullPointerException: Cannot invoke "String.length()" because "this.input" is null
        at jdk.hotspot.agent/sun.jvm.hotspot.RMIHelper.rebind(RMIHelper.java:78)
        at jdk.hotspot.agent/sun.jvm.hotspot.HotSpotAgent.setupDebugger(HotSpotAgent.java:379)
        at jdk.hotspot.agent/sun.jvm.hotspot.HotSpotAgent.go(HotSpotAgent.java:329)
        at jdk.hotspot.agent/sun.jvm.hotspot.HotSpotAgent.startServer(HotSpotAgent.java:215)
        at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runDEBUGD(SALauncher.java:431)
        at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:493)
Caused by: java.lang.NullPointerException: Cannot invoke "String.length()" because "this.input" is null
        at java.base/java.net.URI$Parser.parse(URI.java:3166)
        at java.base/java.net.URI.<init>(URI.java:623)
        at java.rmi/java.rmi.Naming.intParseURL(Naming.java:273)
        at java.rmi/java.rmi.Naming.parseURL(Naming.java:237)
        at java.rmi/java.rmi.Naming.rebind(Naming.java:171)
        at jdk.hotspot.agent/sun.jvm.hotspot.RMIHelper.rebind(RMIHelper.java:64)
        ... 5 more
```