funcr: handle fmt.Stringer that panics on nil

[pohly]

I ran into this with klog in Kubernetes (see kubernetes/klog#278). While I haven't tested it, funcr is probably also affected, at least here but potentially also elsewhere:

logr/funcr/funcr.go

Line 360 in 99e02a9

value = v.String()

The problem is that log calls might pass a nil pointer for a struct which implements fmt.Stringer, but then segfaults when called. Here's an example:
https://github.com/kubernetes/kubernetes/blob/a1e8a5bf39d48719dfbcf49ea09223ee04840502/pkg/kubelet/kubelet.go#L2334-L2335

I think it is reasonable to expect that a logging implementation is resilient against such problems. That means that funcr must either check for nil beforehand or deal with the panic. I'm undecided. Perhaps fmt.Sprintf("%q", <nil stringer>) can serve as reference.