Bluetooth: SMP: Don't try to encrypt with a non-existing LTK
Bluetooth: SMP: Don't try to encrypt with a non-existing LTK

When we receive a security request we need to make sure that any
existing keys contain an LTK to encrypt with. Otherwise there's a risk
of trying to encrypt with an all-zeroes LTK.

Fixes #3221

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

ldts/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,kraj/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr