SecurityPkg/DxeImageVerificationLib: Fix memory leaks (CVE-2019-14575)

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1608

Pointer HashCtx used in IsCertHashFoundInDatabase() is not freed inside
the while-loop, if it will run more than once.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c

@@ -908,6 +908,9 @@ IsCertHashFoundInDatabase (
       goto Done;
     }
 
+    FreePool (HashCtx);
+    HashCtx = NULL;
+
     SiglistHeaderSize = sizeof (EFI_SIGNATURE_LIST) + DbxList->SignatureHeaderSize;
     CertHash          = (EFI_SIGNATURE_DATA *) ((UINT8 *) DbxList + SiglistHeaderSize);
     CertHashCount     = (DbxList->SignatureListSize - SiglistHeaderSize) / DbxList->SignatureSize;