Bluetooth: l2cap: fix null pointer dereference

bt_conn_create_pdu_timeout() may return NULL if no buffer
is available, l2cap_chan_create_seg() does not check the
subsequent return value.
Fix possible null pointer dereference in l2cap_chan_create_seg()
and l2cap_chan_le_send().

Signed-off-by: Johann Fischer <j.fischer@phytec.de>