avformat/hlsenc: Only write something if there is something to write

This is especially important in case avio_write() would be switched
to an unsigned type like size_t, then a potential error from avio_read()
(with negative return value) would no longer be handled gracefully by
avio_write().

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

libavformat/hlsenc.c

@@ -2383,8 +2383,8 @@ static int64_t append_single_file(AVFormatContext *s, VariantStream *vs)
 
     do {
         read_byte = avio_read(vs->out, buf, BUFSIZE);
-        avio_write(vs->out_single_file, buf, read_byte);
         if (read_byte > 0) {
+            avio_write(vs->out_single_file, buf, read_byte);
             total_size += read_byte;
             ret = total_size;
         }