diracdec: use correct buffer for slice_params_buf realloc

This fixes a double-free detected by AddressSanitizer.

The problem was introduced in commit
dcad4677d6.

Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/diracdec.c

@@ -907,7 +907,7 @@ static int decode_lowdelay(DiracContext *s)
     int slice_num = 0;
 
     if (s->slice_params_num_buf != (s->num_x * s->num_y)) {
-        s->slice_params_buf = av_realloc_f(s->thread_buf, s->num_x * s->num_y, sizeof(DiracSlice));
+        s->slice_params_buf = av_realloc_f(s->slice_params_buf, s->num_x * s->num_y, sizeof(DiracSlice));
         if (!s->slice_params_buf) {
             av_log(s->avctx, AV_LOG_ERROR, "slice params buffer allocation failure\n");
             return AVERROR(ENOMEM);