MdeModulePkg: Refine the code for DxeHttpLib

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdeModulePkg/Library/DxeHttpLib/DxeHttpLib.c

@@ -1674,6 +1674,8 @@ HttpGenRequestMessage (
   ASSERT (Message != NULL);
 
   *RequestMsg           = NULL;
+  Status                = EFI_SUCCESS;
+  HttpHdrSize           = 0;
   MsgSize               = 0;
   Success               = FALSE;
   HttpHdr               = NULL;