MdeModulePkg/Variable: Initialize local variable "RtPtrTrack"

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2364

Fixes a new build warning in VS2012 introduced in f8ff4cc.

This patch initializes the local variable "RtPtrTrack" in
FindVariableInRuntimeCache ().

This ensures the pointers in the structure are initialized
in the case no variable stores exist in the list of variable
stores.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.c

@@ -590,6 +590,8 @@ FindVariableInRuntimeCache (
     return EFI_INVALID_PARAMETER;
   }
 
+  ZeroMem (&RtPtrTrack, sizeof (RtPtrTrack));
+
   //
   // The UEFI specification restricts Runtime Services callers from invoking the same or certain other Runtime Service
   // functions prior to completion and return from a previous Runtime Service call. These restrictions prevent