avcodec/cbs_sei: Don't use -1th element of array

(This affected only suffix SEI messages; yet no such SEI messages are
currently inserted.)

Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/cbs_sei.c

@@ -166,7 +166,7 @@ static int cbs_sei_get_unit(CodedBitstreamContext *ctx,
         }
         if (i < 0) {
             // No VCL units; just put it at the end.
-            position = -1;
+            position = au->nb_units;
         } else {
             position = i + 1;
         }