Ensure that the number of burners is zero before incrementing it

When hal is not available, num_burners is set to -1. When detecting drives
with libburn after that, num_burners is incremented on each burner found,
starting at -1 instead of 0, letting xfburn think no burner is available.

--- xfburn-0.4.3.orig/xfburn/xfburn-device-list.c
+++ xfburn-0.4.3/xfburn/xfburn-device-list.c
@@ -308,6 +308,7 @@
   DBG ("After scanning for drives");
 
   priv->num_drives = (gint) num_drives;
+  priv->num_burners = 0;
 
   if (ret < 0)
     g_warning ("An error occurred while scanning for available drives");