nfp: bpf: Add check for nfp_app_ctrl_msg_alloc()

In the Linux kernel, the following vulnerability has been resolved:

nfp: bpf: Add check for nfp_app_ctrl_msg_alloc()

Add check for the return value of nfp_app_ctrl_msg_alloc() in

nfp_bpf_cmsg_alloc() to prevent null pointer dereference.