s3/lib/dbwrap: clang: Fix 'Access to field results in a deref of a null'

s3/lib/dbwrap: clang: Fix 'Access to field results in a deref of a null'

Fixes:

source3/lib/dbwrap/dbwrap_ctdb.c:530:39: warning: Access to field 'm_write' results in a dereference of a null pointer (loaded from field 'transaction') <--[clang]
        if (pull_newest_from_marshall_buffer(ctx->transaction->m_write, key,

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>