lib: rb: Code guideline fixes

Fixes violations code guideline related with essential types rules:

- use explicit comparison with 0 or NULL
- ensure computations are done in the destination precision
- avoid mixing enumerations with integers: when this is unavoidable,
  always convert enums to integers and not the other way around
- avoid mixing signed and unsigned integers in computations and
  comparisons

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
Signed-off-by: Abramo Bagnara <abramo.bagnara@bugseng.com>