src/scanadf.c: Fix segfault when `scanadf -h -d <device>`

scanadf segfaults when printing help for a specific device, because
device handle is closed before `scanadf_exit()`, but not set to NULL.

Fixes Fedora bugzilla 2225209

src/scanadf.c

@@ -1672,7 +1672,10 @@ Type ``%s --help -d DEVICE'' to get list of all options for DEVICE.\n\
 \n\
 List of available devices:", prog_name);
       if (device)
+	{
 	  sane_close(device);
+	  device = NULL;
+	}
 
       status = sane_get_devices (&device_list, SANE_FALSE);
       if (status == SANE_STATUS_GOOD)