Bluetooth: audio: Fix possible memory violation

This fixes possible memory violation by using the index outside of an
instance array by tbs_client_common_call_control.
This basically fixes the get_inst_by_index function to return NULL if
the index is invalid. The function calls have been guarded to catch the
returned NULL.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>