net, init: assert each network reachability is true by default

The default network reachability values are implicitly set
by this line in net.cpp:

static bool vfLimited[NET_MAX] GUARDED_BY(g_maplocalhost_mutex) = {};

This commit asserts that each network is reachable during
the first loop through them during bitcoind init.

src/init.cpp

@@ -1301,6 +1301,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
         for (int n = 0; n < NET_MAX; n++) {
             enum Network net = (enum Network)n;
+            assert(IsReachable(net));
             if (!nets.count(net))
                 SetReachable(net, false);
         }