gspca_sn9c20x: Fix NULL pointer dereference

Don't call v4l2_ctrl_g_ctrl on ctrls which the model cam in question
does not have.
Reported-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[ Taken directly, since Mauro is on vacation ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/media/video/gspca/sn9c20x.c

@@ -2070,8 +2070,11 @@ static int sd_start(struct gspca_dev *gspca_dev)
 	set_gamma(gspca_dev, v4l2_ctrl_g_ctrl(sd->gamma));
 	set_redblue(gspca_dev, v4l2_ctrl_g_ctrl(sd->blue),
 			v4l2_ctrl_g_ctrl(sd->red));
+	if (sd->gain)
 		set_gain(gspca_dev, v4l2_ctrl_g_ctrl(sd->gain));
+	if (sd->exposure)
 		set_exposure(gspca_dev, v4l2_ctrl_g_ctrl(sd->exposure));
+	if (sd->hflip)
 		set_hvflip(gspca_dev, v4l2_ctrl_g_ctrl(sd->hflip),
 				v4l2_ctrl_g_ctrl(sd->vflip));
 
@@ -2176,7 +2179,7 @@ static void sd_dqcallback(struct gspca_dev *gspca_dev)
 	struct sd *sd = (struct sd *) gspca_dev;
 	int avg_lum;
 
-	if (!v4l2_ctrl_g_ctrl(sd->autogain))
+	if (sd->autogain == NULL || !v4l2_ctrl_g_ctrl(sd->autogain))
 		return;
 
 	avg_lum = atomic_read(&sd->avg_lum);