security: initialize transform::Downstream::m_index

This avoids use of uninitialised value in Security/Transform/TestBoolSink/Basic
test case.

refs #3763

Change-Id: If24929d4c85d3a803d3901acb7cc4db6d7cd7798
diff --git a/src/security/transform/transform-base.cpp b/src/security/transform/transform-base.cpp
index 60e967b..b04c3f5 100644
--- a/src/security/transform/transform-base.cpp
+++ b/src/security/transform/transform-base.cpp
@@ -33,6 +33,7 @@
 
 Downstream::Downstream()
   : m_isEnd(false)
+  , m_index(0)
 {
 }