subsys/shell: check for shell context before accessing it
subsys/shell: check for shell context before accessing it

The ASSERT order is backwards, where the shell->ctx is being accessed
before it is being checked as valid.  Moving the check for the
shell->ctx to before the using it.

Signed-off-by: Dan Kalowsky <5fb29269e90e7f59540525cea3545f89343cfd4f@amperecomputing.com>

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