policycoreutils: semodule_package: do not fail with an empty fc file

When running sepolgen tests on a Linux 4.7 kernel, one test fails with
the following message:

    /usr/bin/semodule_package:  Failed to mmap
    tmp/module_compile_test.mod.fc:  Invalid argument

The .fc file is empty, which is why it cannot be used with mmap().
Anyway the current code supports empty files (with if() conditions in
main()) so do not try to mmap an empty file in file_to_data().

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

policycoreutils/semodule_package/semodule_package.c

@@ -72,6 +72,10 @@ static int file_to_data(const char *path, char **data, size_t * len)
 			path, strerror(errno));
 		goto err;
 	}
+	if (!sb.st_size) {
+		*len = 0;
+		return 0;
+	}
 
 	*data = mmap(NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
 	if (*data == MAP_FAILED) {