avformat/mxfenc: use a zero based continuity counter

The standard does not seem to require the counter to be zero based, but some
checker tools (MyriadBits MXFInspect, Interra Baton) have validations against 0
start...

Fixes ticket #6781.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/mxfenc.c

@@ -2686,7 +2686,7 @@ static void mxf_write_system_item(AVFormatContext *s)
     avio_w8(pb, mxf->content_package_rate); // content package rate
     avio_w8(pb, 0x00); // content package type
     avio_wb16(pb, 0x00); // channel handle
-    avio_wb16(pb, (mxf->tc.start + frame) & 0xFFFF); // continuity count, supposed to overflow
+    avio_wb16(pb, frame & 0xFFFF); // continuity count, supposed to overflow
     if (mxf->essence_container_count > 1)
         avio_write(pb, multiple_desc_ul, 16);
     else {

tests/ref/fate/mxf-user-comments

-683bacb0105e5bc5bbf46aa430c644d1
+c6469c0ae2aaee602eacbc009080ae8e

tests/ref/lavf/mxf

-649009e3d3d62eb3b6c56334d057cc4d *tests/data/lavf/lavf.mxf
+27b98795036b334e100c15c7e06d948f *tests/data/lavf/lavf.mxf
 526393 tests/data/lavf/lavf.mxf
 tests/data/lavf/lavf.mxf CRC=0x8dddfaab
-9076b7015cffe8aa72883e900a2041a5 *tests/data/lavf/lavf.mxf
+783b475a818602f54e947094d57e2981 *tests/data/lavf/lavf.mxf
 561721 tests/data/lavf/lavf.mxf
 tests/data/lavf/lavf.mxf CRC=0x96ff1b48
 02bf8f0cd8951a49e277306691cb1538 *tests/data/lavf/lavf.mxf