auth: fix a crash issue due to CryptoHandler::create() failed

auth: fix a crash issue due to CryptoHandler::create() failed

In this case(e.g. user passes wrong key), attempts to call the CryptoKey.ckh will lead to a segfault.

This patch fixes crash issue like following:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffed10e700 (LWP 25051)]
0x00007ffff59896c6 in CryptoKey::encrypt (this=0x7fffed10d4f0, cct=0x555555829c30, in=..., out=..., error=0x7fffed10d440) at auth/cephx/../Crypto.h:110
110	    return ckh->encrypt(in, out, error);
(gdb) bt
    at auth/cephx/../Crypto.h:110
    at auth/cephx/CephxProtocol.h:464

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>