LeAudio: Fix unsuported offload capabilities config

In a case where a json file configurations for a particualar
context do not match any of the offloader capabilities, we
were returning a random memory address as an audio set configuration
pointer.

Bug: 295972694
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine bluetooth_le_audio_codec_manager_test
Change-Id: I03d22725e081cac45df4bbeada929e626093ab2f