ntdll: fix leak in test_symlinks

[Dan Kegel]

Fixes the little test leak found by
http://kegel.com/wine/valgrind/logs/2010-02-12-20.25/diff-ntdll_reg.txt

78 bytes in 1 blocks are definitely lost in loss record 318 of 545
   at notify_alloc (heap.c:254)
   by RtlAllocateHeap (heap.c:1700)
   by test_symlinks (reg.c:619)
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