LeAudio: Fix crash on active group deactivation

If there are multiple calls scheduled on the main loop, it might happen
that the active LE Audio group is deactivated while the state machine
goes to streaming state. We should handle it gracefully rather than assert.

Bug: 338937189
Test: atest bluetooth_le_audio_client_test
Flag: Exempt; trivial fix, unit tested
Change-Id: I893694717035983fb079377752fb1e9f89e9b8d7