CryptoPkg BaseCryptLib: Init the content of struct 'CertCtx' before use

Some fields in structure 'CertCtx' might be used uninitialized in function
Pkcs7GetCertificatesList().

This commit makes sure that 'CertCtx' gets initialized before being used.

Cc: Long Qin <qin.long@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Qin Long <qin.long@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

CryptoPkg/Library/BaseCryptLib/Pk/CryptPkcs7Verify.c

@@ -10,7 +10,7 @@
   WrapPkcs7Data(), Pkcs7GetSigners(), Pkcs7Verify() will get UEFI Authenticated
   Variable and will do basic check for data structure.
 
-Copyright (c) 2009 - 2015, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2009 - 2016, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -489,6 +489,8 @@ Pkcs7GetCertificatesList (
   OldBuf         = NULL;
   Signers        = NULL;
 
+  ZeroMem (&CertCtx, sizeof (CertCtx));
+
   //
   // Parameter Checking
   //