dix: Fix undefined memset in dixInitPrivates

When we set these up initially, no subsystems have allocated any
privates yet, so the storage address will be null, and then:

    #0  0x000000000040cb30 in __ubsan_handle_nonnull_arg_abort@plt ()
    #1  0x00000000005f6f88 in _dixInitPrivates (privates=0x103f140, addr=0x0,
        type=PRIVATE_CLIENT) at ../dix/privates.c:450
    #2  0x00000000005f784e in dixAllocatePrivates (privates=0x103f140,
        type=PRIVATE_CLIENT) at ../dix/privates.c:522
    #3  0x000000000056d5a8 in dix_main (argc=2, argv=0x7fffffffdd28,
        envp=0x7fffffffdd40) at ../dix/main.c:176
    #4  0x000000000041704e in main (argc=2, argv=0x7fffffffdd28,
        envp=0x7fffffffdd40) at ../dix/stubmain.c:34