net: sockets: tls: Check accepted context is not NULL
net: sockets: tls: Check accepted context is not NULL

When closing a listening socket the functions waiting on the
FIFO will be unblocked this will result in receiving a NULL child
context. If that is the case return an error instead of carrying on.
Return the same error code (EINVAL) that is returned on Linux when
calling shutdown on a blocked accept call.

Signed-off-by: Léonard Bise <de3916488e017c874a84534423a3bc0001130d5f@gmail.com>

galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr