MdeModulePkg: Add ASSERT to make sure pointer 'OptionalData' not be NULL

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

MdeModulePkg/Library/BootMaintenanceManagerUiLib/Variable.c

@@ -614,6 +614,7 @@ Var_UpdateDriverOption (
 
   if (OptionalDataExist) {
     NewLoadContext->OptionalData = AllocateZeroPool (LoadOption.OptionalDataSize);
+    ASSERT (NewLoadContext->OptionalData != NULL);
     CopyMem (
       NewLoadContext->OptionalData,
       LoadOption.OptionalData,
@@ -725,6 +726,7 @@ Var_UpdateBootOption (
 
   if (OptionalDataExist) {
     NewLoadContext->OptionalData = AllocateZeroPool (LoadOption.OptionalDataSize);
+    ASSERT (NewLoadContext->OptionalData != NULL);
     CopyMem (
       NewLoadContext->OptionalData,
       LoadOption.OptionalData,