QCli: Fix crash with mkv attachments #707

Job	Run time
build-macos-brew-Qt6 (clang-9)	3m 42s
build-linux (gcc-9)	7m 3s
build-macos-brew (clang-9)	2m 49s
build-linux-Qt6 (gcc-9)	8m 18s
build-macos-no-brew (clang-9)	4m 34s
build-macos-no-brew-Qt6 (clang-9)	5m 33s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	11m 23s
build-windows-vs (5.15.*)	8m 13s
build-windows-mingw (5.15.*)	9m 29s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	13m 3s
	1h 14m 7s

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