crypto: ccp - Silence may-be-uninitialized warning in sev_ioctl_do_pdh_export

The recent reordering of code in sev_ioctl_do_pdh_export triggered
a false-positive may-be-uninitialized warning from gcc:

In file included from ../include/linux/sched/task.h:13,
                 from ../include/linux/sched/signal.h:9,
                 from ../include/linux/rcuwait.h:6,
                 from ../include/linux/percpu-rwsem.h:7,
                 from ../include/linux/fs.h:34,
                 from ../include/linux/compat.h:17,
                 from ../arch/x86/include/asm/ia32.h:7,
                 from ../arch/x86/include/asm/elf.h:10,
                 from ../include/linux/elf.h:6,
                 from ../include/linux/module.h:19,
                 from ../drivers/crypto/ccp/sev-dev.c:11:
In function ‘copy_to_user’,
    inlined from ‘sev_ioctl_do_pdh_export’ at ../drivers/crypto/ccp/sev-dev.c:2036:7,
    inlined from ‘sev_ioctl’ at ../drivers/crypto/ccp/sev-dev.c:2249:9:
../include/linux/uaccess.h:225:16: warning: ‘input_cert_chain_address’ may be used uninitialized [-Wmaybe-uninitialized]
  225 |         return _copy_to_user(to, from, n);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/crypto/ccp/sev-dev.c: In function ‘sev_ioctl’:
../drivers/crypto/ccp/sev-dev.c:1961:22: note: ‘input_cert_chain_address’ was declared here
 1961 |         void __user *input_cert_chain_address;
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~

Silence it by moving the initialisation of the variables in question
prior to the NULL check.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>