libceph: fix crypto key null deref, memory leak

(cherry picked from commit f0666b1ac875ff32fe290219b150ec62eebbe10e)

Avoid crashing if the crypto key payload was NULL, as when it was not correctly
allocated and initialized.  Also, avoid leaking it.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>