security: Initialise connection handle to an invalid handle

The value 0 being in the 0x0000 to 0x0EFF range
is a valid Bluetooth connection handle.
Instead we initialise the connection handle with an invalid
value of 0xFFFF.

This bug was triggered by RootCanal as it gives for it's
third connection a 0 connection handle.

Test: gd/cert/run SecurityTest
Change-Id: I16e1c17d9db0cb2c2b8f6099d3d669d0793e41b4