Bluetooth: CAP: Broadcast: Add check for memory allocation for create

When creating a broadcast source with
bt_cap_initiator_broadcast_audio_create there was no check if
all broadcast sources were already allocated, which could cause
a NULL pointer dereference.

Add a check, a test and documentation about possibly
error codes of the function.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>