core: fix value of buffer variable "num_history" when the value defined in option weechat.history.max_commands is reached

src/gui/gui-history.c

@@ -88,7 +88,7 @@ gui_history_buffer_add (struct t_gui_buffer *buffer, const char *string)
                     free (buffer->last_history->text);
                 free (buffer->last_history);
                 buffer->last_history = ptr_history;
-                buffer->num_history++;
+                buffer->num_history--;
             }
         }
     }