avformat/dashenc, hlsenc: Return 0 on succes from write_header

Do not propagate the return value of avformat_write_header(),
as it contains the information whether the output had
already been initialized in avformat_init_output(),
but this is set generically; the return value of
FFOutputFormat.write_header is not documented at all
(and is currently ignored if >= 0), but it is more future-proof
to simply return 0 on success.

Reviewed-by: Liu Steven <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/dashenc.c

@@ -1747,7 +1747,7 @@ static int dash_write_header(AVFormatContext *s)
             (ret = flush_init_segment(s, os)) < 0)
             return ret;
     }
-    return ret;
+    return 0;
 }
 
 static int add_segment(OutputStream *os, const char *file,

libavformat/hlsenc.c

@@ -2372,7 +2372,7 @@ static int hls_write_header(AVFormatContext *s)
         }
     }
 
-    return ret;
+    return 0;
 }
 
 static int hls_init_file_resend(AVFormatContext *s, VariantStream *vs)