MdeModulePkg/BootManagerMenu: Add assertion to indicate no DIV by 0

BootMenuSelectItem() contains code to DIV BootMenuData->ItemCount.
When BootMenuData->ItemCount can be 0, the DIV operation may
trigger CPU exception.
But in logic, this case won't happen. So add assertion to indicate
it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>