jpegparse: fix incorrect reading of transform in app14 marker
"adobe" in app14 marker seem not a null-terminted string. so, when we use gst_byte_reader_get_string_utf8, more bytes will be read until null. and "gst_byte_reader_get_uint8 (&reader, &transform)" will almost fail to read transform
this is my test file, and i find jpegparse consider it is CMYK, but YCCK by libjpeg-turbo
and from source code, ffmpeg/libjpeg-turbo seem do not consider "Adobe" a null-terminated string.
ffmpeg parsing app14 flow:
https://github.com/FFmpeg/FFmpeg/commit/0f31d401c35c6d39462c2b3e2aa7b573db0ccfde
libjpeg-turbo emit jpeg app14:
but this PDF consider it is null-terminted