netapi32: Set the buffer to NULL in NetApiBufferFree

[Kai Blin]

This fixes a Valgrind warning triggered by the NetApiBufferFree() test
where a buffer is free()d twice.
---
 dlls/netapi32/apibuf.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
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