refactor(bot.zig): deinit all backlog slots

Previously we deinited backwards from the most recently heard message
until we reach a null-slot.

Now this code has been simplified to walk the backlog and deinit any
found messages.

src/bot.zig

@@ -171,12 +171,9 @@ pub const Bot = struct {
         }
         self.sent_messages.deinit(self.allocator);
 
-        var idx = self.previous_idx(self.top);
-        while (idx != self.bottom) : (idx = self.previous_idx(idx)) {
-            if (self.backlog[idx]) |message| {
+        for (self.backlog) |slot| {
+            if (slot) |message| {
                 message.deinit(self.allocator);
-            } else {
-                break;
             }
         }
     }