Bug 468431

SUMMARY
When rendering an animation as a gif, it fails with the error message:

Could not render animation:
Unknown error.

I logged the output of ffmpeg using a wrapper script and it reports these errors:

Reading option '-start_number' ...Unrecognized option 'start_number'.
Error splitting the argument list: Option not found
Unrecognized option 'start_number'.
Error splitting the argument list: Option not found

STEPS TO REPRODUCE
1.  New custom document (I did 64x64, but I don't think that matters)
2.  Add a blank frame in the animation view
3. Try to render animation as a gif (ffmpeg is installed of course)

OBSERVED RESULT
Error window:
Could not render animation:
Unknown error.

EXPECTED RESULT
Renders animation to gif file

SOFTWARE/OS VERSIONS
ArchLinux
Krita Version 5.1.5 (flatpak install)
Running on gnome (xorg)
ffmpeg version (archlinux repo):
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libjxl --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-opencl --enable-opengl --enable-shared --enable-version3 --enable-vulkan
libavutil      58.  2.100 / 58.  2.100
libavcodec     60.  3.100 / 60.  3.100
libavformat    60.  3.100 / 60.  3.100
libavdevice    60.  1.100 / 60.  1.100
libavfilter     9.  3.100 /  9.  3.100
libswscale      7.  1.100 /  7.  1.100
libswresample   4. 10.100 /  4. 10.100
libpostproc    57.  1.100 / 57.  1.100