libselinux: set errno on label lookup failure #519

Job	Run time
build (gcc, 3.11, 3.1)	2m 5s
build (gcc, 3.11, 3.1, test-flags-override)	1m 51s
build (gcc, 3.11, 3.1, test-debug)	1m 50s
build (gcc, 3.11, 3.1, linker-bfd)	2m 28s
build (gcc, 3.11, 3.1, linker-gold)	2m 32s
build (gcc, 3.10, 3.1)	2m 37s
build (gcc, 3.9, 3.1)	2m 25s
build (gcc, 3.8, 3.1)	2m 13s
build (gcc, 3.7, 3.1)	2m 6s
build (gcc, pypy3.7, 3.1)	3m 16s
build (gcc, 3.11, 3.0)	2m 12s
build (gcc, 3.11, 2.7)	2m 16s
build (gcc, 3.11, 2.6)	2m 38s
build (gcc, 3.11, 2.5)	2m 32s
build (clang, 3.11, 3.1)	2m 12s
build (clang, 3.11, 3.1, test-flags-override)	1m 58s
build (clang, 3.11, 3.1, test-debug)	1m 56s
build (clang, 3.10, 3.1)	2m 36s
build (clang, 3.9, 3.1)	3m 11s
build (clang, 3.8, 3.1)	2m 24s
build (clang, 3.7, 3.1)	2m 16s
build (clang, pypy3.7, 3.1)	3m 33s
build (clang, 3.11, 3.0)	2m 42s
build (clang, 3.11, 2.7)	2m 20s
build (clang, 3.11, 2.6)	2m 25s
build (clang, 3.11, 2.5)	2m 27s
build (gcc, 3.11, 3.1, sanitizers)	2m 31s
	1h 5m 32s

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