Fix crash on reconnection in the chat window

Profanity tries to access the nickname from the roster,
but roster is being cleaned already,
thus leading to use-after-free.

Fix #1894 #1894

src/ui/window.c

@@ -312,7 +312,10 @@ win_get_title(ProfWin* window)
     {
         const ProfChatWin* chatwin = (ProfChatWin*)window;
         assert(chatwin->memcheck == PROFCHATWIN_MEMCHECK);
-        const PContact contact = roster_get_contact(chatwin->barejid);
+        PContact contact = NULL;
+        if (roster_exists()) {
+            contact = roster_get_contact(chatwin->barejid);
+        }
         if (!contact) {
             return g_strdup(chatwin->barejid);
         }