cap_rights_clear() with no additional arguments acts as a no-op, so
instead of removing all capability rights from STDIN_FILENO, the same
rights were allowed for STDIN_FILENO as were allowed for src_fd.
Fixes: a0eecc235d3b ("xz: Make Capsicum sandbox more strict with stdin and stdout.")
(The commit message says "stdout". It should have said "stderr".)