latest build crashes when cropdetect is selected

[dericed]

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[dericed]

I bisected and found this bug is caused by 46905cf, which adds a new graph with 3 plotted values lavfi.signalstats.YBITDEPTH, lavfi.signalstats.UBITDEPTH, and lavfi.signalstats.VBITDEPTH.

This issue can be quickly seen via:

ffmpeg -f lavfi -i testsrc -f lavfi -i sine -t 0.25 hey.mkv
./qctools-cli/qcli -f cropdetect+idet -i hey.mkv

with a result of

qcli dev build, Oct 31 2020 10:57:24
filters selected: signalstats cropdetect idet 

analyzing input file... hey.mkv
                                                   0 of 100 %zsh: segmentation fault  ./qctools-cli/qcli -f signalstats+cropdetect+idet -i hey.mkv

I do note that if I remove 1 of the 3 added plots (YBITDEPTH, UBITDEPTH, or VBITDEPTH) then it works.

I narrowed the crash scenario down to the combination of cropdetect and idet. Perhaps over combinations are found, but these two seem required to reproduce it.