fix small problem with derefing null nscomptr... the compiler should know what I mean, not what I type!

widget/src/windows/nsFilePicker.cpp

@@ -52,6 +52,8 @@ nsFilePicker::nsFilePicker()
   mNumberOfFilters = 0;
   mUnicodeEncoder = nsnull;
   mUnicodeDecoder = nsnull;
+  mDisplayDirectory = do_CreateInstance("component://mozilla/file/local");
+
 }
 
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