callbacks, Misc, Relay: Fix when the message prefix is just a nick

<user> and <host> are optional in https://datatracker.ietf.org/doc/html/rfc1459#section-2.3.1

Closes GH-1451.

plugins/Misc/plugin.py

@@ -381,7 +381,10 @@ class Misc(callbacks.Plugin):
         If <nick> is given, it takes the continuation of the last command from
         <nick> instead of the person sending this message.
         """
-        userHostmask = msg.prefix.split('!', 1)[1]
+        if '!' in msg.prefix and '@' in msg.prefix:
+            userHostmask = msg.prefix.split('!', 1)[1]
+        else:
+            userHostmask = msg.nick
         if nick:
             try:
                 (private, L) = irc._mores[nick]

plugins/Relay/plugin.py

@@ -341,7 +341,7 @@ class Relay(callbacks.Plugin):
         if channel not in self.registryValue('channels'):
             return
         network = self._getIrcName(irc)
-        if self.registryValue('hostmasks', channel):
+        if self.registryValue('hostmasks', channel) and '!' in msg.prefix:
             hostmask = format(' (%s)', msg.prefix.split('!')[1])
         else:
             hostmask = ''
@@ -355,7 +355,7 @@ class Relay(callbacks.Plugin):
         if channel not in self.registryValue('channels'):
             return
         network = self._getIrcName(irc)
-        if self.registryValue('hostmasks', channel):
+        if self.registryValue('hostmasks', channel) and '!' in msg.prefix:
             hostmask = format(' (%s)', msg.prefix.split('!')[1])
         else:
             hostmask = ''

src/callbacks.py

@@ -883,8 +883,9 @@ class ReplyIrcProxy(RichReplyMethods):
                     prefix = state.nickToHostmask(target)
                 except KeyError:
                     pass # We'll leave it as it is.
-            mask = prefix.split('!', 1)[1]
-            self._mores[mask] = msgs
+            if '!' in prefix and '@' in prefix:
+                mask = prefix.split('!', 1)[1]
+                self._mores[mask] = msgs
             public = bool(self.msg.channel)
             private = kwargs.get('private', False) or not public
             self._mores[msg.nick] = (private, msgs)