btm_acl_removed: Check for null as acl may have already closed am: ed4c4715...

btm_acl_removed: Check for null as acl may have already closed am: ed4c4715 am: 94112aaa am: 7071ae60

Original change: https://android-review.googlesource.com/c/platform/system/bt/+/1474376

Change-Id: I3e421aec569b81a7c562f7293e86e59a98963558