avformat/assenc: do not copy null terminator

The `par->extradata` buffer filled from some matroska files may be
null terminated, and use `ffio_write_lines` using the full buffer
length will copy this null character into the output files. This
results in a file in which there is a null terminator after the
header, but preceeding the actual content of the subtitle file.
Treat this buffer as a string and write line with the text length of this buffer to skip the null character.

Regression from 7bf1b9b

debian/patches/0082-fix-ass-incorrect-null-copy.patch

@@ -0,0 +1,14 @@
+Index: FFmpeg/libavformat/assenc.c
+===================================================================
+--- FFmpeg.orig/libavformat/assenc.c
++++ FFmpeg/libavformat/assenc.c
+@@ -67,7 +67,8 @@ static int write_header(AVFormatContext
+                 ass->trailer = trailer;
+         }
+
+-        ffio_write_lines(s->pb, par->extradata, header_size, NULL);
++        header_size = av_strnlen(par->extradata, header_size);
++        ffio_write_lines(s->pb, par->extradata, (int)header_size, NULL);
+
+         ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]");
+         if (!strstr(par->extradata, "\n[Events]"))

debian/patches/series

@@ -79,3 +79,4 @@
 0079-videotoolbox-remove-opengl-compatability.patch
 0080-use-dynamic-pool-for-vpl-qsv-hwupload.patch
 0081-backport-av1-videotoolbox.patch
+0082-fix-ass-incorrect-null-copy.patch