shell32: Don't use the working directory passed into ShellExecute* functions if it isn't valid.

[Robert Shearman]

---
  dlls/shell32/shlexec.c |   20 +++++++++++++++-----
  1 files changed, 15 insertions(+), 5 deletions(-)

An example of a program that does this is Outlook 2003. Using a test 
program shows that passing garbage in succeeds on Windows.
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