auth: fix NULL pointer access when trying to delete CryptoAESKeyHandler instance

auth: fix NULL pointer access when trying to delete CryptoAESKeyHandler instance

the caller needs to check the nullity of the parameter before calling
PK11_FreeSymKey or PK11_FreeSlot, otherwise if CryptoAESKeyHandler::init
failed, we will hit a segfault as follows:
  #0  0x00007f76844f5a95 in PK11_FreeSymKey () from /lib64/libnss3.so
  #1  0x00007f76586b6e49 in CryptoAESKeyHandler::~CryptoAESKeyHandler() () from /lib64/librados.so.2
  #2  0x00007f76586b5eea in CryptoAES::get_key_handler(ceph::buffer::ptr const&, std::string&) () from /lib64/librados.so.2
  #3  0x00007f76586b4b9c in CryptoKey::_set_secret(int, ceph::buffer::ptr const&) () from /lib64/librados.so.2
  #4  0x00007f76586b4e95 in CryptoKey::decode(ceph::buffer::list::iterator&) () from /lib64/librados.so.2
  #5  0x00007f76586b7ee6 in KeyRing::set_modifier(char const*, char const*, EntityName&, std::map<std::string, ceph::buffer::list, std::less<std::string>, std::allocator<std::pair<std::string const, ceph::buffer::list> > >&) () from /lib64/librados.so.2
  #6  0x00007f76586b8882 in KeyRing::decode_plaintext(ceph::buffer::list::iterator&) () from /lib64/librados.so.2
  #7  0x00007f76586b9803 in KeyRing::decode(ceph::buffer::list::iterator&) () from /lib64/librados.so.2
  #8  0x00007f76586b9a1f in KeyRing::load(CephContext*, std::string const&) () from /lib64/librados.so.2
  #9  0x00007f76586ba04b in KeyRing::from_ceph_context(CephContext*) () from /lib64/librados.so.2
  #10 0x00007f765852d0cd in MonClient::init() () from /lib64/librados.so.2
  #11 0x00007f76583c15f5 in librados::RadosClient::connect() () from /lib64/librados.so.2
  #12 0x00007f765838cb1c in rados_connect () from /lib64/librados.so.2
  ...

Signed-off-by: runsisi <runsisi@zte.com.cn>