[patch] remove crashes in putenv(): use "" instead of NULL value

[Jarosław Staniek]

For review; remove crashes in putenv(): use "" instead of NULL value (needed 
e.g. for unsetenv()).

NULL values caused a crash in strlen() and _putenv_s(), at least on Vista.

Noticed that in the code of knotify4:

   unsetenv( "SESSION_MANAGER" );
-- 
regards / pozdrawiam, Jaroslaw Staniek
  Sponsored by OpenOffice Polska (http://www.openoffice.com.pl/en) to work on
  Kexi & KOffice (http://www.kexi.pl/en, http://www.koffice.org/kexi)
  KDE Libraries for MS Windows (http://windows.kde.org)
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