ffmetadata decoding handles lines ending with backslashes incorrectly

[Michael Lelli]

Summary of the bug:
In ffmetadata files, if a line ends with an escaped backslash the next line return is also escaped when it should not be. This only happens when using Unix newlines (\n). Using Windows newlines (\r\n) does not show this issue.

How to reproduce:
Create this ffmpeg tag file with Unix newlines:

;FFMETADATA1
test1=qwer\\
test2=asdf

Write, then read the metadata from a file:

ffmpeg -y -i in.mp4 -i meta.txt -map_metadata 1 -movflags use_metadata_tags -codec copy -f mp4 out.mp4

ffmpeg -i out.mp4 -f ffmetadata -

Expected results (and results with Windows newlines):

;FFMETADATA1
...
test1=qwer\\
test2=asdf

Actual results:

;FFMETADATA1
...
test1=qwer\\\
test2\=asdf

Version:

ffmpeg version 2021-05-02-git-5541cffa17-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint

[Michael Lelli]

Nevermind, appears to be a dup of #9144