user32/tests/dde.c: allocate big enough buffer for UnpackDDElParam test

[Dan Kegel]

Looks like buffer was just too small?  James confirms this patch look ok.

Fixes heap corruption warning
err:heap:HEAP_ValidateTail Heap 0x7f000000: invalid arena 0x7f0ed960,
buffer 0x7f0ed968, corrupt at 0x7f0ed96a (0 bytes past end of 2 byte
buffer)
and valgrind warning
Invalid write of size 4
   at  test_UnpackDDElParam (dde.c:2018)
   by  func_dde (dde.c:2429)
   by  run_test (test.h:535)
   by  main (test.h:585)
 Address 0x7f0ed968 is 0 bytes inside a block of size 2 alloc'd
   at  notify_alloc (heap.c:247)
   by  RtlAllocateHeap (heap.c:1697)
   by  HeapAlloc (heap.c:276)
   by  GlobalAlloc (heap.c:369)
   by  test_UnpackDDElParam (dde.c:2016)
   by  func_dde (dde.c:2429)
   by  run_test (test.h:535)
   by  main (test.h:585)
seen in http://kegel.com/wine/valgrind/logs/2009-11-30-19.16/vg-user32_dde.txt
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