dix: Fix undefined memset in _dixInitScreenPrivates

Again, memset(0, ...) is undefined:

    #0  0x000000000040cb30 in __ubsan_handle_nonnull_arg_abort@plt ()
    #1  0x00000000005f954a in _dixInitScreenPrivates (pScreen=0x114d6b0,
        privates=0x13b8c80, addr=0x0, type=PRIVATE_PICTURE)
        at ../dix/privates.c:687
    #2  0x00000000005f9941 in _dixAllocateScreenObjectWithPrivates (
        pScreen=0x114d6b0, baseSize=120, clear=120, offset=80,
        type=PRIVATE_PICTURE) at ../dix/privates.c:722
    #3  0x0000000000745b2f in CreatePicture (pid=2097154, pDrawable=0x13b8ae0,
        pFormat=0x1083ba0, vmask=0, vlist=0x13b0934, client=0x10564c0,
        error=0x7fffffffdb14) at ../render/picture.c:749
    #4  0x0000000000751bf4 in ProcRenderCreatePicture (client=0x10564c0)
        at ../render/render.c:588
    #5  0x000000000075e425 in ProcRenderDispatch (client=0x10564c0)
        at ../render/render.c:1983
    #6  0x0000000000535db4 in Dispatch () at ../dix/dispatch.c:498
    #7  0x000000000056de21 in dix_main (argc=2, argv=0x7fffffffdd28,
        envp=0x7fffffffdd40) at ../dix/main.c:274
    #8  0x000000000041704e in main (argc=2, argv=0x7fffffffdd28,
        envp=0x7fffffffdd40) at ../dix/stubmain.c:34