crypto: Always initialize splitkeylen

crypto: Always initialize splitkeylen

When _FORTIFY_SOURCE=2, glibc version is 2.35, and GCC version is
12.1.0, the compiler complains as follows:

In file included from /usr/include/string.h:535,
                 from /home/alarm/q/var/qemu/include/qemu/osdep.h:99,
                 from ../crypto/block-luks.c:21:
In function 'memset',
    inlined from 'qcrypto_block_luks_store_key' at ../crypto/block-luks.c:843:9:
/usr/include/bits/string_fortified.h:59:10: error: 'splitkeylen' may be used uninitialized [-Werror=maybe-uninitialized]
   59 |   return __builtin___memset_chk (__dest, __ch, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   60 |                                  __glibc_objsize0 (__dest));
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~
../crypto/block-luks.c: In function 'qcrypto_block_luks_store_key':
../crypto/block-luks.c:699:12: note: 'splitkeylen' was declared here
  699 |     size_t splitkeylen;
      |            ^~~~~~~~~~~

It seems the compiler cannot see that splitkeylen will not be used
when splitkey is NULL. Suppress the warning by initializing splitkeylen
even when splitkey stays NULL.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>