SH-3328 llcorehttp tracing control needed defensive logic for missing setting

Guard for the typical case of a missing setting with a bracketing
gSavedSettings.controlExists() call.

indra/newview/llappcorehttp.cpp

@@ -85,10 +85,14 @@ void LLAppCoreHttp::init()
 	// 1 - Basic start, stop simple transitions
 	// 2 - libcurl CURLOPT_VERBOSE mode with brief lines
 	// 3 - with partial data content
-	long trace_level(0L);
-	trace_level = long(gSavedSettings.getU32("QAModeHttpTrace"));
-	status = LLCore::HttpRequest::setPolicyGlobalOption(LLCore::HttpRequest::GP_TRACE, trace_level);
-
+	static const std::string http_trace("QAModeHttpTrace");
+	if (gSavedSettings.controlExists(http_trace))
+	{
+		long trace_level(0L);
+		trace_level = long(gSavedSettings.getU32(http_trace));
+		status = LLCore::HttpRequest::setPolicyGlobalOption(LLCore::HttpRequest::GP_TRACE, trace_level);
+	}
+	
 	// Setup default policy and constrain if directed to
 	mPolicyDefault = LLCore::HttpRequest::DEFAULT_POLICY_ID;
 	static const std::string texture_concur("TextureFetchConcurrency");