control/zone: Use const references when iterating

We don't want to copy the object out of the container we are iterating
over.

Tested:
    Built and run through unit tests.

Change-Id: I68b041f98e1c6a6a9dbc59448b629e1d22ba62c9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/control/zone.cpp b/control/zone.cpp
index d37701e..7b19acc 100644
--- a/control/zone.cpp
+++ b/control/zone.cpp
@@ -419,7 +419,7 @@
 {
     for (auto it = _timerEvents.begin(); it != _timerEvents.end(); ++it)
     {
-        auto teEventData = *std::get<timerEventDataPos>(*it);
+        const auto& teEventData = *std::get<timerEventDataPos>(*it);
         if (std::get<eventActionsPos>(teEventData).size() ==
             eventActions.size())
         {