bluetooth: Check for hcon during during sock_close

It is possible that during BT OFF operation the hcon
could be released from a tasklet context while we are
trying to send the l2cap disconnect req. Make sure
that hcon is valid before dereferencing it during
l2cap disconnect req.

CRs-fixed: 383345
Change-Id: Icb12c62560013b5ebb047c1c5d4bfe04b3a793ef
Signed-off-by: Sunny Kapdi <sunnyk@codeaurora.org>