lavfi/graphdump: Replace the number by macro for bprint init

Replace the number by macro for bprint init.
Signed-off-by: Jun Zhao <mypopydev@gmail.com>

libavfilter/graphdump.c

@@ -156,7 +156,7 @@ char *avfilter_graph_dump(AVFilterGraph *graph, const char *options)
     AVBPrint buf;
     char *dump;
 
-    av_bprint_init(&buf, 0, 0);
+    av_bprint_init(&buf, 0, AV_BPRINT_SIZE_COUNT_ONLY);
     avfilter_graph_dump_to_buf(&buf, graph);
     av_bprint_init(&buf, buf.len + 1, buf.len + 1);
     avfilter_graph_dump_to_buf(&buf, graph);