Multi-line topic messages aren’t handled correctly

Description of the issue

When an XMPP user tries to set a topic containing a \n, biboumi fails to detect it and sends it as is, creating invalid commands.

Relevant debug logs

Apr 06 11:02:55 fenouil biboumi[27995]: xmpp/xmpp_component.cpp:159:        XMPP RECEIVING: <message from='louiz@coucou/adb3e486'  to='#biboumi%bla@irc.louiz.org' type='groupchat'><subject xmlns='jabber:component:accept'>chat
Apr 06 11:02:55 fenouil biboumi[27995]: coucou</subject></message>
Apr 06 11:02:55 fenouil biboumi[27995]: src/irc/irc_client.cpp:381:        IRC SENDING: (yozora-irc.net) IrcMessage[TOPIC]{#biboumi}{chat
Apr 06 11:02:55 fenouil biboumi[27995]: coucou}
Apr 06 11:02:55 fenouil biboumi[27995]: src/irc/irc_client.cpp:343:        IRC RECEIVING: (yozora-irc.net) IrcMessage[TOPIC]{#biboumi}{chat}(from: louiz!louiz@Yozora-E524149C.louiz.org)
Apr 06 11:02:55 fenouil biboumi[27995]: xmpp/xmpp_component.cpp:77:        XMPP SENDING: <message from='#biboumi%bla@irc.louiz.org/louiz' to='louiz@coucou/adb3e486' type='groupchat'><subject>chat</subject></message>
Apr 06 11:02:55 fenouil biboumi[27995]: src/irc/irc_client.cpp:343:        IRC RECEIVING: (yozora-irc.net) IrcMessage[421]{louiz}{coucou}{Unknown command}(from: Yozora-irc.net)