avformat/hlsenc: remove the first slash of the relative path line in the master m3u8 file

Please testing with the following command:
./ffmpeg -y -i input.mkv \
 -b✌️0 5250k -c:v h264 -pix_fmt yuv420p -profile:v main -level 4.1 \
 -b🅰️0 256k \
 -c:a mp2 -ar 48000 -ac 2 -map 0:v -map 0🅰️0\
 -f hls -var_stream_map "v:0,a:0" \
 -master_pl_name master.m3u8 -t 300 -hls_time 10 -hls_init_time 4 -hls_list_size \
 10 -master_pl_publish_rate 10  -hls_flags \
 delete_segments+discont_start+split_by_time ./tmp/video.m3u8

then cat ./tmp/master.m3u8
before:
#EXTM3U
#EXT-X-VERSION:3
#EXT-X-STREAM-INF:BANDWIDTH=6056600,RESOLUTION=1280x720,CODECS="avc1.4d4829,mp4a.40.33"
/video.m3u8

$ ./ffmpeg -i  ./tmp/master.m3u8 -c:v copy -c:a mp2 ./test.mkv
[hls @ 0x7f82f9000000] Skip ('#EXT-X-VERSION:3')
[hls @ 0x7f82f9000000] Opening '/video.m3u8' for reading
[hls @ 0x7f82f9000000] parse_playlist error No such file or directory [/video.m3u8]
./tmp/master.m3u8: No such file or directory

after:
#EXTM3U
#EXT-X-VERSION:3
#EXT-X-STREAM-INF:BANDWIDTH=6056600,RESOLUTION=1280x720,CODECS="avc1.4d4829,mp4a.40.33"
video.m3u8

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

libavformat/hlsenc.c

@@ -1256,7 +1256,7 @@ static const char* get_relative_url(const char *master_url, const char *media_ur
         }
     }
 
-    return media_url + base_len;
+    return media_url + base_len + 1;
 }
 
 static int64_t get_stream_bit_rate(AVStream *stream)