NTB: variable dereferenced before check

commit 186f27ff upstream.

Correct instances of variable dereferencing before checking its value on
the functions exported to the client drivers.  Also, add sanity checks
for all exported functions.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>