libselinux: set errno on label lookup failure

selabel_lookup(3) is expected to set errno on lookup failure, i.e. when
it returns NULL.  Set errno if a regular expression failed to compile in
compile_regex().

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

libselinux/src/label_file.h

@@ -407,6 +407,7 @@ static inline int compile_regex(struct spec *spec, const char **errbuf)
 			*errbuf = &regex_error_format_buffer[0];
 		}
 		__pthread_mutex_unlock(&spec->regex_lock);
+		errno = EINVAL;
 		return -1;
 	}