MdeModulePkg/EhciPei: Initialize the variable Map

Map is used but not Initialized.
Map is called by IoMmuMap, in which Mapping(Map) is called by IoMmu->Map.
We can not assume Map is given an initial value at any step.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>

MdeModulePkg/Bus/Pci/EhciPei/EhciUrb.c

@@ -534,6 +534,8 @@ EhcCreateUrb (
   PEI_URB                       *Urb;
   VOID                          *Map;
 
+  Map = NULL;
+
   Urb = Ehc->Urb;
   Urb->Signature  = EHC_URB_SIG;
   InitializeListHead (&Urb->UrbList);