tools/intel_vbt_decode: fix division by zero child device size

Real world VBTs keep fuzzing our decoder, this time with a legacy child
devices block #11 that has child_dev_size 0, leading to division by
zero. Check for it, and bail out early, both for legacy and current
child device blocks.

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>