cve-update-db-native: skip on empty cpe23Uri

Recently an entry in the NVD DB appeared that looks like that
{'vulnerable': True, 'cpe_name': []}.
As besides all the vulnerable flag no data is present we would get
a KeyError exception on acccess.
Use get method on dictionary and return if no meta data is present
Also quit if the length of the array after splitting is less than 6

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>