Safe last MUC message timestamp per MUC

After pasis review of my code he thinks it's better to safe the
timestamp per MUC so we can account for some problems that could occur
with timing.

src/ui/mucwin.c

@@ -54,6 +54,8 @@ mucwin_new(const char *const barejid)
     ProfWin *window = wins_new_muc(barejid);
     ProfMucWin *mucwin = (ProfMucWin *)window;
 
+    mucwin->last_msg_timestamp = NULL;
+
 #ifdef HAVE_OMEMO
     if (muc_anonymity_type(mucwin->roomjid) == MUC_ANONYMITY_TYPE_NONANONYMOUS && omemo_automatic_start(barejid)) {
         omemo_start_muc_sessions(barejid);