cilium: make error message, not segfault
cilium: make error message, not segfault

In (a rare) case when there is no BPF filesystem mounted the cilium client will
segfault on an attempt to display filesystem information:

    # cilium bpf fs show
    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xbeef000]
    ...

This patch fixes this case and now the client will just print an error message:

    # cilium bpf fs show
    Error: No BPF filesystems are mounted

Signed-off-by: Anton Protopopov <302f9ad3a2e1cbf999e415016f4fb86fd4f39c95@isovalent.com>

michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium