[media] gspca: reset image_len to 0 on LAST_PACKET when discarding frame

Reset image and image_len to NULL/0 on LAST_PACKET when we're in discard
frame mode, just like we do when not discarding the current frame.

The new se401 driver uses image_len for SOF/EOF detection and thus depends on
this.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/gspca/gspca.c

@@ -443,8 +443,11 @@ void gspca_frame_add(struct gspca_dev *gspca_dev,
 	} else {
 		switch (gspca_dev->last_packet_type) {
 		case DISCARD_PACKET:
-			if (packet_type == LAST_PACKET)
+			if (packet_type == LAST_PACKET) {
 				gspca_dev->last_packet_type = packet_type;
+				gspca_dev->image = NULL;
+				gspca_dev->image_len = 0;
+			}
 			return;
 		case LAST_PACKET:
 			return;