fftools/ffmpeg: Replace the number by macro for bprint init

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

fftools/ffmpeg.c

@@ -1689,7 +1689,7 @@ static void print_report(int is_last_report, int64_t timer_start, int64_t cur_ti
 
     vid = 0;
     av_bprint_init(&buf, 0, AV_BPRINT_SIZE_AUTOMATIC);
-    av_bprint_init(&buf_script, 0, 1);
+    av_bprint_init(&buf_script, 0, AV_BPRINT_SIZE_AUTOMATIC);
     for (i = 0; i < nb_output_streams; i++) {
         float q = -1;
         ost = output_streams[i];