shadow_copy2: revert expensive and unnecessary zero-initialization

shadow_copy2: revert expensive and unnecessary zero-initialization

I was being overly cautious. This is initialization is not
necessary, since further down in the for-loop, the memory
always gets fully initialized because the insert string is
inserted at various slash positions.

So this talloc_zero_array can be skipped: this an expensive
thing to do in virtually every VFS call.

This essentially reverts commit 249e9b4a34d8959bd94735c1921ecfc24d6a2705.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 71318d7d685f8e1e112076f84ef2d788acb83547)