network: validate server peers sent by main server

Data returned by the main server for request "server.peers.subscribe"
is of course untrusted input. Previously if it contained e.g. invalid port numbers
or IP addresses, it could kill the whole network taskgroup.
(this might have only affected master and not released versions,
which would only raise exceptions once the client actually tried to connect to an invalid host/port)

electrum/network.py

@@ -77,7 +77,9 @@ NUM_RECENT_SERVERS = 20
 
 
 def parse_servers(result: Sequence[Tuple[str, str, List[str]]]) -> Dict[str, dict]:
-    """ parse servers list into dict format"""
+    """Convert servers list (from protocol method "server.peers.subscribe") into dict format.
+    Also validate values, such as IP addresses and ports.
+    """
     servers = {}
     for item in result:
         host = item[1]
@@ -89,6 +91,7 @@ def parse_servers(result: Sequence[Tuple[str, str, List[str]]]) -> Dict[str, dic
                 if re.match(r"[st]\d*", v):
                     protocol, port = v[0], v[1:]
                     if port == '': port = constants.net.DEFAULT_PORTS[protocol]
+                    ServerAddr(host, port, protocol=protocol)  # check if raises
                     out[protocol] = port
                 elif re.match("v(.?)+", v):
                     version = v[1:]
@@ -431,6 +434,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
             random.shuffle(server_peers)
             max_accepted_peers = len(constants.net.DEFAULT_SERVERS) + NUM_RECENT_SERVERS
             server_peers = server_peers[:max_accepted_peers]
+            # note that 'parse_servers' also validates the data (which is untrusted input!)
             self.server_peers = parse_servers(server_peers)
             self.notify('servers')
         async def get_relay_fee():