Bug 1346196 - URLCLASSIFIER_UPDATE_REMOTE_STATUS2 records values using empty keys. r=francois

mTelemetryProvider might be empty if |CancelUpdate| is called during update.
In that scenario, the call flow will look like:
CancelUpdate -> UpdateError -> DownloadDone -> set mTelemetryProvider to empty ->
OnStartRequest -> record telemetry with empty key.

So in this patch, move mTelemetryProvider.Truncate() to OnStopRequest.

MozReview-Commit-ID: JDgNUqR4SKa

--HG--
extra : rebase_source : 42aa4c898f403a1189e67e355246ebed5aa4af44

toolkit/components/url-classifier/nsUrlClassifierStreamUpdater.cpp

@@ -99,7 +99,6 @@ nsUrlClassifierStreamUpdater::DownloadDone()
   mSuccessCallback = nullptr;
   mUpdateErrorCallback = nullptr;
   mDownloadErrorCallback = nullptr;
-  mTelemetryProvider.Truncate();
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -781,6 +780,7 @@ nsUrlClassifierStreamUpdater::OnStopRequest(nsIRequest *request, nsISupports* co
     rv = mDBService->FinishUpdate();
   }
 
+  mTelemetryProvider.Truncate();
   mChannel = nullptr;
 
   // If the fetch failed, return the network status rather than NS_OK, the