MdeModulePkg/BDS: Fix a buffer overflow bug

MdeModulePkg/BDS: Fix a buffer overflow bug

KeyOption points to a buffer holding the content of Key####.
So its size is smaller than EFI_BOOT_MANAGER_KEY_OPTION.
Old code to assign value to KeyOption->OptionNumber modifies
the memory outside of the KeyOption buffer.

The patch fixes this bug.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Steven Shi <steven.shi@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>