avcodec/cbs_lcevc: append with position -1 in ff_cbs_lcevc_add_process_block()

ff_cbs_lcevc_list_add() resolves a position of -1 to the end of the
list, but the block was then set up at blocks[-1].

Fixes: Am7oZsN0qfFR/testcase/cbs_lcevc_append.c
Fixes: Am7oZsN0qfFR
Fixes: out of array write
Out of array write Replicated through API with ASAN

Found during the review of PR 25408.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_lcevc.c

@@ -628,6 +628,9 @@ int ff_cbs_lcevc_add_process_block(CodedBitstreamContext *ctx,
     if (err < 0)
         return err;
 
+    if (position == -1)
+        position = list->nb_blocks;
+
     // Add a new block to the message list.
     err = ff_cbs_lcevc_list_add(list, position);
     if (err < 0)