CA-106080: Guard List.assoc "sku_type" license_params with mem_assoc

From code review on pull request #1226: since sku_type is a key that's returned
from v6d, and not present natively in xapi, we need to test that sku_type is
present before calling assoc on license_params.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>