Don't return pointer on freed memory on error.

PR:		http://code.google.com/p/nfc-tools/issues/detail?id=87
Submitted by:	jkmalinen@gmail.com

libnfc/drivers/pn53x_usb.c

@@ -430,6 +430,7 @@ pn53x_usb_open (const nfc_connstring connstring)
 error:
   // Free allocated structure on error.
   nfc_device_free (pnd);
+  pnd = NULL;
 free_mem:
   free (desc.dirname);
   free (desc.filename);