cmake: avoid `-Wnonnull` warning in `HAVE_FSETXATTR_5` detection #1661

Job	Run time
cygwin, AM x86_64 openssl R	9m 54s
cygwin, CM x86_64 openssl	6m 57s
linux-mingw, AM gcc	2m 14s
dl-mingw, CM 9.5.0-x86_64 schannel	8m 57s
linux-mingw, CM gcc	1m 20s
dl-mingw, CM 7.3.0-x86_64 schannel U	14m 23s
msvc, CM x64-windows schannel MultiSSL U	7m 38s
dl-mingw, CM 6.4.0-i686 schannel !unity	3m 17s
msvc, CM x64-windows openssl	9m 26s
msvc, CM x64-uwp openssl	3m 44s
msys2, AM x86_64 !proxy	7m 11s
mingw32ce, AM 4.4.0-arm schannel	2m 6s
msvc, CM x64-windows libressl	8m 40s
msys2, AM x86_64 default	4m 55s
mingw32ce, CM 4.4.0-arm schannel	1m 35s
msvc, CM x64-windows boringssl	7m 33s
msys2, CM x86_64 default	5m 49s
msvc, CM x64-windows wolfssl	8m 1s
msys2, AM x86_64 default R	8m 4s
msvc, CM x64-windows mbedtls	9m 33s
mingw, AM x86_64 default	6m 26s
mingw, AM x86_64 c-ares U	10m 26s
mingw, CM x86_64 schannel c-ares U	7m 5s
mingw, CM ucrt-x86_64 schannel R TrackMemory	2m 36s
mingw, CM clang-x86_64 openssl	2m 46s
mingw, CM ucrt-x86_64 schannel uwp	2m 46s
mingw, CM x86_64 schannel dev debug	2m 25s
mingw, CM i686 schannel R	2m 29s
	2h 48m 16s

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