ntb: fix possible NULL dereference

kmalloc can fail and we should check for NULL before using the pointer
returned by kmalloc.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>