Refine code to make it more safely.

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

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15597 6f19259b-4bc3-4df7-8a09-765794883524

PerformancePkg/Library/TscTimerLib/DxeTscTimerLib.c

@@ -59,11 +59,13 @@ DxeTscTimerLibConstructor (
   EFI_STATUS  Status;
   UINT64      *TscFrequency;
 
+  TscFrequency = NULL;
   //
   // Get TSC frequency from system configuration table with TSC frequency GUID.
   //
   Status = EfiGetSystemConfigurationTable (&gEfiTscFrequencyGuid, (VOID **) &TscFrequency);
   if (Status == EFI_SUCCESS) {
+    ASSERT (TscFrequency != NULL);
     mTscFrequency = *TscFrequency;
     return EFI_SUCCESS;
   }