Fix #3950: Div by 0 during DXT4 DDS load #4345

Job	Run time
macos-11 appleclang13/C++17 py3.9 MacOS-11 boost1.76 exr3.1 ocio2.1	25m 40s
macos-12 appleclang13/C++20 py3.10 MacOS-12 boost1.76 exr3.1 ocio2.1	35m 9s
macos-13 appleclang14/C++20 py3.11 MacOS-13 boost1.76 exr3.1 ocio2.1	13m 25s
windows-2019 VS2019	44m 15s
VFX2019 gcc6/C++14 py2.7 boost1.66 exr2.4 ocio1.1	21m 43s
VFX2020 gcc6/C++14 py3.7 boost1.70 exr2.4 ocio1.1	14m 5s
VFX2021 gcc9/C++17 py3.7 boost1.73 exr2.5 ocio2.0	19m 44s
VFX2021 clang10/C++14 avx2 exr2.5 ocio2.0	22m 42s
VFX2022 gcc9/C++17 py39 boost1.76 exr3.1 ocio2.1	11m 40s
VFX2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	17m 46s
VFX2022 icc/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	24m 58s
VFX2022 icx/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	29m 58s
VFX2022 sanitizers	39m 41s
VFX2023 gcc11/C++17 py3.10 boost1.80 exr3.1 ocio2.2	15m 30s
VFX2019 oldest/hobbled gcc6.3/C++14 py2.7 boost-1.66 exr-2.4 no-sse no-ocio	8m 6s
VFX2022 clang-format	5m 41s
Ubuntu latest releases gcc11 C++17 avx2 exr3.1 ocio2.2	12m 36s
Ubuntu bleeding edge gcc12 C++20 py3.10 OCIO/libtiff/exr-master boost1.74 avx2	13m 11s
Ubuntu clang14 C++20 avx2 exr3.1 ocio2.1	28m 41s
Ubuntu debug gcc7/C++14, sse4.2, exr2.4	17m 50s
Ubuntu gcc8 C++17 avx exr2.5	27m 10s
Ubuntu static libs gcc7 C++14 exr2.4	11m 58s
	7h 41m 29s

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