The UEFI specification does not say anything when the pointers passed to

The UEFI specification does not say anything when the pointers passed to
WaitForEvent() are NULL.
Passing NULL pointer would cause a segmentation fault in the current code.
This change prevents to get segmentation faults in this case.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15643 6f19259b-4bc3-4df7-8a09-765794883524