avcodec/dvbsubdec: Add const where appropriate

These pointers sometimes point to static storage (namely to
default_clut), so adding const to the pointed-to type is important to
ensure that one does not accidentally modify something that is not owned
by a single AVCodecContext.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/dvbsubdec.c

@@ -724,8 +724,8 @@ static int save_subtitle_set(AVCodecContext *avctx, AVSubtitle *sub, int *got_ou
     DVBSubDisplayDefinition *display_def = ctx->display_definition;
     DVBSubRegion *region;
     AVSubtitleRect *rect;
-    DVBSubCLUT *clut;
-    uint32_t *clut_table;
+    const DVBSubCLUT *clut;
+    const uint32_t *clut_table;
     int i;
     int offset_x=0, offset_y=0;
     int ret = 0;
@@ -1443,8 +1443,8 @@ static int save_display_set(DVBSubContext *ctx)
 {
     DVBSubRegion *region;
     DVBSubRegionDisplay *display;
-    DVBSubCLUT *clut;
-    uint32_t *clut_table;
+    const DVBSubCLUT *clut;
+    const uint32_t *clut_table;
     int x_pos, y_pos, width, height;
     int x, y, y_off, x_off;
     uint32_t *pbuf;