libext2fs: always refuse to open a file system with a zero s_desc_size #216

Job	Run time
Build and test with clang	3s
Build and test with gcc	24s
Build mke2fs on Windows with mingw32	3s
Build and test on macOS	4s
Build and test with gcc -m32	55s
Build and test with UBSAN enabled	1m 38s
Build mke2fs on Windows with mingw64	2s
Cross-compile for Android (aarch64)	2s
Cross-compile for Android (armv7a)	5m 3s
Build and test with ASAN enabled	1m 53s
Cross-compile for Android (i686)	7s
Cross-compile for Android (x86_64)	4s
	10m 18s

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