userenv: tests/userenv.c: fix buffer overrun

[Dan Kegel]

Found via valgrind.  Fixes

 Use of uninitialised value of size 4
    at wine_cp_mbstowcs (mbtowc.c:79)
    by MultiByteToWideChar (locale.c:1846)
    by CompareStringA (locale.c:2722)
    by get_env (userenv.c:140)
    by func_userenv (userenv.c:218)
  Uninitialised value was created by a stack allocation
    at get_env (userenv.c:128)