Elasticsearch version: 6.4.2

Plugins installed: []

JVM version (java -version): 1.8.0_191

OS version (uname -a if on a Unix-like system): Linux 4.15.0-38-generic #41-Ubuntu SMP Wed Oct 10 10:59:38 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:

When adding an invalid license, a buffer underflow error results. I presume that the cause is some part of the license being invalid, but that the error could/should be better caught and the response message could be clearer.
The error resulted from using the example here without putting in the actual license.

Steps to reproduce:

1. Put the license:

PUT _xpack/license
{
  "licenses": [
    {
      "uid":"893361dc-9749-4997-93cb-802e3d7fa4xx",
      "type":"basic",
      "issue_date_in_millis":1411948800000,
      "expiry_date_in_millis":1914278399999,
      "max_nodes":1,
      "issued_to":"issuedTo",
      "issuer":"issuer",
      "signature":"xx"
    }
    ]
}

2. Results in response:

{
  "error": {
    "root_cause": [
      {
        "type": "buffer_underflow_exception",
        "reason": null
      }
    ],
    "type": "buffer_underflow_exception",
    "reason": null
  },
  "status": 500
}