qdrawhelper: (attempt to) fix Coverity warning about `op` being unini… #17464

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Job	Run time
build (ubuntu-20.04, ubuntu-20.04, libgl-dev libglu-dev 'libxcb*-dev' libx11-xcb-dev libxkbcommon...	2s
build (macos-10.15, macos-10.15, jpeg sqlite libpng pcre2 harfbuzz freetype libb2 double-conversi...	0s
build (windows-2019, windows-2019, choco install, --yes --no-progress, ninja, -qt-sqlite -qt-pcre...	4s
build (ubuntu-18.04, ubuntu-18.04, libgl-dev libglu-dev 'libxcb*-dev' libx11-xcb-dev libxkbcommon...	0s
	6s

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