Dead code in API rmw_create_subscription()
becasue the condition "rmw_subscription" cannot be true in the
"fail" clauses, the execution cannot reach this statement:

rmw_subscription_free(rmw_subscription);

finally, the "rmw_subscribtion" is free by
rmw_destroy_subscription() when it's not a nullptr

Dead code in API rmw_create_publisher()
because the condition "rmw_publisher" cannot be true in the
"fail" clauses, the execution cannot reach this statement:

rmw_publisher_free(rmw_publisher);

finally, the "rmw_publisher" is free by
rmw_destroy_publisher() when it's not a nullptr

Signed-off-by: Ethan Gao <[email protected]>

this memory allocation checking avoids the unintended
behaviour which derives from unknown memory free issue
at rmw_destroy_publisher

Signed-off-by: Ethan Gao <[email protected]>

Signed-off-by: Ethan Gao <[email protected]>

Signed-off-by: Chris Lalancette <[email protected]>