Windows: call MIDL_user_allocate instead of calloc

Windows: call MIDL_user_allocate instead of calloc

In the RPC service routines do not call calloc() directly.
All memory will be deallocated by a call to MIDL_user_free()
so use MIDL_user_allocate() to perform the allocation.

Modify MIDL_user_allocate() to call calloc() instead of malloc()
to ensure that the memory is initialized to NUL bytes.

Reviewed-on: http://gerrit.openafs.org/8365
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 8c7846f4e1dd89db4ff3b03c558d97cd13cbb205)

Change-Id: I5562dbac4977a5d23407cd62c3deb743e629c48c
Reviewed-on: http://gerrit.openafs.org/8645
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>