diracdec: clear slice_params_num_buf on allocation failure

Otherwise it can be non-zero next time decode_lowdelay is called, causing
slice_params_buf not to be allocated, leading to a NULL pointer dereference.

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

@@ -910,6 +910,7 @@ static int decode_lowdelay(DiracContext *s)
         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");
+            s->slice_params_num_buf = 0;
             return AVERROR(ENOMEM);
         }
         s->slice_params_num_buf = s->num_x * s->num_y;