demux/avformat: fix return-value of IORead (fixes #17574)

av_read_frame expects 0 to be returned on end-of-file, and negative
values are reserved for fatal stream-errors. The previous
implementation would return -1 upon EOF (vlc_stream_Read returning 0),
causing premature EOF from modules/demux/avformat/demux.c:Demux.

These changes make sure that we honor the contract associated with the
read-callback, and that we only return -1 if there is a fatal error.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/demux/avformat/demux.c

@@ -1119,7 +1119,7 @@ static int IORead( void *opaque, uint8_t *buf, int buf_size )
     demux_t *p_demux = opaque;
     if( buf_size < 0 ) return -1;
     int i_ret = vlc_stream_Read( p_demux->s, buf, buf_size );
-    return i_ret ? i_ret : -1;
+    return i_ret >= 0 ? i_ret : -1;
 }
 
 static int64_t IOSeek( void *opaque, int64_t offset, int whence )