manage_sru script can't handle un-recognized kernel

[Po-Hsu Lin]

There is a list in the script:
RELEASES = {'2.6': 'lucid',
            '3.2': 'precise',
            '3.13': 'trusty',
            '3.16': 'utopic',
            '3.19': 'vivid',
            '3.13lts': 'precise',
            '3.16lts': 'trusty',
            '3.19lts': 'trusty',
            '4.2lts': 'trusty',
            '4.4lts': 'trusty',
            '4.2': 'wily',
            '4.4': 'xenial'}

If the kernel number is not in this dictionary (yakkety for this case), the script will crash like:

Bug #1640928 in Kernel SRU Workflow: "linux-snapdragon: 4.4.0-1036.40 -proposed tracker": https://bugs.launchpad.net/kernel-sru-workflow/+bug/1640928
Bug #1641180 in Kernel SRU Workflow: "linux-raspi2: 4.8.0-1019.22 -proposed tracker": https://bugs.launchpad.net/kernel-sru-workflow/+bug/1641180
kernel file: /home/certification/sru-kernels/snapdragon-xenial_kernel updated
Traceback (most recent call last):
  File "/home/certification/hwcert-tools/sru_tools/manage_sru", line 277, in <module>
    sys.exit(main())
  File "/home/certification/hwcert-tools/sru_tools/manage_sru", line 274, in main
    args.func(**params)
  File "/home/certification/hwcert-tools/sru_tools/manage_sru", line 72, in notify_kernels
    codename = kernel_name.split('-')[1] + '-' + RELEASES.get(codename)
TypeError: coercing to Unicode: need string or buffer, NoneType found