MdeModulePkg UefiBootManagerLib: Fix VS2012 build failure

Initialize local variable Description as NULL first.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

MdeModulePkg/Library/UefiBootManagerLib/BmBoot.c

@@ -2213,6 +2213,7 @@ BmRegisterBootManagerMenu (
   UINTN                              DataSize;
 
   DevicePath = NULL;
+  Description = NULL;
   //
   // Try to find BootMenuApp from LoadFile protocol
   //