avformat/hlsenc: fix fmp4_init_filename file name with %v problem

when set option fmp4_init_filename to init_%v.mp4
before patch:
the init file will be init_%v_0.mp4, init_%v_1.mp4
after patch:
the init file will be init_0.mp4, init_1.mp4
Reported-By: Gyan Doshi <ffmpeg@gyani.pro>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/hlsenc.c

@@ -2740,7 +2740,11 @@ static int hls_init(AVFormatContext *s)
                 av_strlcpy(vs->fmp4_init_filename, hls->fmp4_init_filename,
                            fmp4_init_filename_len);
                 if (hls->nb_varstreams > 1) {
-                    ret = append_postfix(vs->fmp4_init_filename, fmp4_init_filename_len, i);
+                    if (av_stristr(vs->fmp4_init_filename, "%v")) {
+                        format_name(vs->fmp4_init_filename, fmp4_init_filename_len, i);
+                    } else {
+                        ret = append_postfix(vs->fmp4_init_filename, fmp4_init_filename_len, i);
+                    }
                     if (ret < 0)
                         goto fail;
                 }