Changes

Osmocom_CTRL: counters: return -1 for absent objects

Teach the CTRL functions that retrieve rate counters to return counters
of -1 when a requested object is not present, instead of erroring and
stopping the test. Like this we can expect absence of an object.

Upcoming patch Ibec009203d38f65714561b7c28edbdbd8b34e704 uses this.

Related: SYS#6773
Change-Id: Id00bbfe721d453e5f551e6d47e1b1d03d219700b