guard against device that went missing in claim/release interface

- add test for missing devices in claim or release
- changlog entry for all the sanei_usb work

ChangeLog

@@ -1,3 +1,13 @@
+2013-03-24 Stéphane Voltz <stef.dev@free.fr>
+	backend/genesys.c: make use of the new sanei_usb_scan_devices function
+	in sane_get_devices.
+
+2013-03-24 Stéphane Voltz <stef.dev@free.fr>
+	* sanei/sanei_usb.c include/sane/sanei_usb.h:
+	separate device search logic from usb init by creating a
+	sanei_usb_scan_devices. Add a sanei_usb_exit function to free
+	allocated resources by sanei_usb_init.
+
 2013-03-21 Rolf Bensch <rolf at bensch hyphen online dot de>
 	* backend/pixma_mp150.c, doc/descriptions/pixma.desc, doc/sane-pixma.man:
 	New scanner Canon Pixma MP230 reported by Petar Dodev.

sanei/sanei_usb.c

@@ -2899,6 +2899,11 @@ sanei_usb_claim_interface (SANE_Int dn, SANE_Int interface_number)
 	   dn);
       return SANE_STATUS_INVAL;
     }
+  if (devices[dn].missing)
+    {
+      DBG (1, "sanei_usb_claim_interface: device dn=%d is missing\n", dn);
+      return SANE_STATUS_INVAL;
+    }
 
   DBG (5, "sanei_usb_claim_interface: interface_number = %d\n", interface_number);
 
@@ -2962,7 +2967,11 @@ sanei_usb_release_interface (SANE_Int dn, SANE_Int interface_number)
 	   dn);
       return SANE_STATUS_INVAL;
     }
-
+  if (devices[dn].missing)
+    {
+      DBG (1, "sanei_usb_release_interface: device dn=%d is missing\n", dn);
+      return SANE_STATUS_INVAL;
+    }
   DBG (5, "sanei_usb_release_interface: interface_number = %d\n", interface_number);
 
   if (devices[dn].method == sanei_usb_method_scanner_driver)