shared/bap: Fix crash detaching streams

shared/bap: Fix crash detaching streams

If a stream is being detached but bt_bap reference is already 0 don't
attempt to detach the stream as they would be freed anyway:

Invalid read of size 8
   at 0x19A360: bap_free (bap.c:2576)
   by 0x19A360: bt_bap_unref (bap.c:2735)
   by 0x19A360: bt_bap_unref (bap.c:2727)
   by 0x160E9A: test_teardown (test-bap.c:412)
   by 0x1A8BCA: teardown_callback (tester.c:434)
 Address 0x55e05e0 is 16 bytes inside a block of size 160 free'd
   at 0x48480E4: free (vg_replace_malloc.c:872)
   by 0x1AD5F6: queue_foreach (queue.c:207)
   by 0x19A1C5: bt_bap_detach (bap.c:3879)
   by 0x19A1C5: bt_bap_detach (bap.c:3855)
   by 0x19A33F: bap_free (bap.c:2574)