Starting switch two times does not crash (#810)

libp2p/switch.nim

@@ -296,6 +296,10 @@ proc stop*(s: Switch) {.async, public.} =
 proc start*(s: Switch) {.async, gcsafe, public.} =
   ## Start listening on every transport
 
+  if s.started:
+    warn "Switch has already been started"
+    return
+
   trace "starting switch for peer", peerInfo = s.peerInfo
   var startFuts: seq[Future[void]]
   for t in s.transports:

tests/testswitch.nim

@@ -1016,3 +1016,13 @@ suite "Switch":
     expect LPError:
       await switch.start()
     # test is that this doesn't leak
+
+  asyncTest "starting two times does not crash":
+    let switch = newStandardSwitch(
+      addrs = @[MultiAddress.init("/ip4/0.0.0.0/tcp/0").tryGet()]
+    )
+
+    await switch.start()
+    await switch.start()
+
+    await allFuturesThrowing(switch.stop())