cmp_kem.c: fix return in case of error #1465

Job	Run time
cygwin (windows-2019, win64, -DCMAKE_C_COMPILER=gcc --strict-warnings no-fips)	37m 1s
minimal (windows-2019)	14m 7s
minimal (windows-2022)	18m 25s
plain (windows-2019)	25m 31s
plain (windows-2022)	26m 42s
shared (windows-2019, win64, enable-fips)	45m 19s
shared (windows-2019, win32, --strict-warnings no-fips)	40m 3s
shared (windows-2022, win64, enable-fips)	41m 9s
shared (windows-2022, win32, --strict-warnings no-fips)	43m 35s
	4h 51m 52s

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