lib/flow: Skip minimask value checks.

lib/flow: Skip minimask value checks.

We allow zero 'values' in a miniflow for it to have the same map
as the corresponding minimask.  Minimasks themselves never have
zero data values, though.  Document this and optimize the code
accordingly.

v2:
- Made miniflow_get_map_in_range() to return data offset instead of
  a pointer via the last parameter.
- Simplified minimatch_hash_in_range() by removing pointer arithmetic.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>