dell1600n_net: return empty list when local devices are requested

!502 made the dell1600n_net backend return GOOD status when local
devices are requested. This is problematic since in that case
the caller may assume that the passed device list has been populated.
However, this is not the case for this backend, since the device list
is not modified in any way in that case. The caller trying to access
the device list may lead to undesired consequences. Fix that by
returning an empty list if 'local_only' is true.

backend/dell1600n_net.c

@@ -233,6 +233,9 @@ static void JpegDecompTermSource (j_decompress_ptr cinfo);
 /* Results of last call to sane_get_devices */
 static struct DeviceRecord *gKnownDevices[MAX_SCANNERS];
 
+/* Empty list for when network devices are not wanted */
+static const SANE_Device *gEmptyDeviceList[1];
+
 /* Array of open scanner device states.
    :NOTE: (int)SANE_Handle is an offset into this array */
 static struct ScannerState *gOpenScanners[MAX_SCANNERS];
@@ -306,15 +309,17 @@ sane_get_devices (const SANE_Device *** device_list, SANE_Bool local_only)
   const char *pVal;
   int valLen;
 
+  if (local_only) {
+    *device_list = gEmptyDeviceList;
+    return SANE_STATUS_GOOD;
+  }
+
   /* init variables */
   ret = SANE_STATUS_GOOD;
   sock = 0;
   pDevice = NULL;
   optYes = 1;
 
-  if (local_only)
-    return ret;
-
   InitComBuf (&queryPacket);
 
   /* clear previous results */