datpaath-windows: consolidate check for output buffer

datpaath-windows: consolidate check for output buffer

Earlier, output buffer was optional in a transaction command. Thus each
command handler, had to check if the output buffer had indeed been
specified by userspace or not.

Now that output buffer is mandatory in transaction command, let's
consolidate the check in one place, and also convert the previous check
to ASSERTs.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>