tests: kernel: mbox_api: fix uninit variable and unchecked value

"struct k_mbox_msg mmsg" should be initialized before using.
"int" value returned by k_mem_pool_alloc() should be checked.

Coverity-CID: 160083
Coverity-CID: 160470

Change-Id: I35714bf9d76723c5fdd8c2963bf76b42ae1b1867
Signed-off-by: Sharron LIU <sharron.liu@intel.com>