Bluetooth: Resolved a race condition

Due to simultaneous disconnects from HCI and SCO
this race condtion arrives. While doing SCO disconnect
a null pointer check for HCI connection should be present
to avoid any crash due to this condtion.

CRs-Fixed: 415887
Change-Id: I23f0a7a256e267650db0abc2fc510b964b64c50c
Signed-off-by: Nitin Srivastava <nitinsr@codeaurora.org>