NTB: ntb_tool: Add check for devm_kcalloc

As the devm_kcalloc may return NULL pointer,
it should be better to add check for the return
value, as same as the others.

Fixes: 7f46c8b3

 ("NTB: ntb_tool: Add full multi-port NTB API support")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>