avcodec/cbs_h266_syntax_template: Don't omit unused function parameter

The calls to the sei_decoded_picture_hash read and write functions
are performed with four pointer arguments; just because one
of them is unused by the callees does not mean that they
can be omitted: This is undefined behaviour.
(This was not recognized because the SEI_MESSAGE_RW macro
contains casts.)

Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 484e7716bc)

libavcodec/cbs_h266_syntax_template.c

@@ -3431,7 +3431,7 @@ static int FUNC(slice_header) (CodedBitstreamContext *ctx, RWContext *rw,
 static int FUNC(sei_decoded_picture_hash) (CodedBitstreamContext *ctx,
                                            RWContext *rw,
                                            H266RawSEIDecodedPictureHash *
-                                           current)
+                                           current, SEIMessageState *unused)
 {
     int err, c_idx, i;