USER32: Avoid crashing in +relay if SetDeskWallPaper is called with -1.

[Mike McCormack]

Not sure why the (LPSTR)-1 isn't caught in the NTDLL_dbgstr_an exception 
handler.  Probably better to fix that, but here's my workaround anyway.

---

  dlls/user/desktop.c   |    9 +++++++++
  dlls/user/user32.spec |    2 +-
  2 files changed, 10 insertions(+), 1 deletions(-)

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