tests: kernel: mbox: fix uninitialized scalar variable of coverity
tests: kernel: mbox: fix uninitialized scalar variable of coverity

Given an zero initialized value before using struct k_mbox_msg, to fix
CID: 219511.
Fix #32934

Signed-off-by: Enjia Mai <233e651694b7770d0f980a273a8e4a6f7e2a84d4@intel.com>

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