fix crash while checking misra_9 when initializing a union with {{0,0}}

[ev1313]

This fixes a crash with following error:

Traceback (most recent call last):
  File "/usr/local/share/Cppcheck/addons/runaddon.py", line 8, in <module>
    runpy.run_path(addon, run_name='__main__')
  File "<frozen runpy>", line 291, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/local/share/Cppcheck/addons/misra.py", line 4737, in <module>
    main()
  File "/usr/local/share/Cppcheck/addons/misra.py", line 4679, in main
    checker.parseDump(item)
  File "/usr/local/share/Cppcheck/addons/misra.py", line 4335, in parseDump
    self.executeCheck(902, self.misra_9_2, cfg)
  File "/usr/local/share/Cppcheck/addons/misra.py", line 4246, in executeCheck
    check_function(*args)
  File "/usr/local/share/Cppcheck/addons/misra.py", line 2104, in misra_9_2
    misra_9.misra_9_x(self, data, 902)
  File "/usr/local/share/Cppcheck/addons/misra_9.py", line 414, in misra_9_x
    parser.parseInitializer(ed, eq.astOperand2)
  File "/usr/local/share/Cppcheck/addons/misra_9.py", line 320, in parseInitializer
    child = self.root.getChildByValueElement(self.ed)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'getChildByValueElement'

A minimal example and testcase is added.
The extra check for nextChild seems to fix it, however i did not read the whole codebase, so maybe this creates other issues.

[danmar]

Thanks a lot for this fix! I am very sorry for the slow reply. I just have a few small nits hope you want to fix it.

[danmar]

If my small nits are fixed I believe it's fine to merge this.

[ev1313]

I updated accordingly