TunBuilderCapture: use default route metric if none was specified

When the user specifies no metric (i.e. value is -1), the TunBuilder
should pass the default value down the stack.

Signed-off-by: Antonio Quartulli <[email protected]>

openvpn/tun/builder/capture.hpp

@@ -489,7 +489,7 @@ class TunBuilderCapture : public TunBuilderBase, public RC<thread_unsafe_refcoun
         Route r;
         r.address = address;
         r.prefix_length = static_cast<unsigned char>(prefix_length);
-        r.metric = metric;
+        r.metric = (metric < 0 ? route_metric_default : metric);
         r.ipv6 = ipv6;
         add_routes.push_back(r);
         return true;