net/mqtt: Remove length computations for some msg fields

Currently, for the following MQTT msg fields:

- client_id
- will_topic
- user_name
- topic

their length is computed inside the routine that receives the MQTT msg.

Although this simplifies development, also imposes one restriction:
data must be null-terminated. Sometimes, data is received from other
sources and not generated by the application, so the null-terminated
constraint may be considered problematic for the user.

This patch removes the assumption that string fields are null-terminated.
Current data structures are already prepared to handle this case, so no
API change is required.

Change-Id: I5a147a5b21e0da49541cbe62baac363c8737cd3e
Signed-off-by: Flavio Santes <flavio.santes@intel.com>