check for nil ctx in *RPCContext.Certificate

Endpoints that are re-used are registered without a `ctx` object, so a
call to *RPCContext.Certificate could cause a panic if used in one of
these endpoints.

nomad/rpc.go

@@ -109,7 +109,7 @@ type RPCContext struct {
 
 // Certificate returns the first certificate available in the chain.
 func (ctx *RPCContext) Certificate() *x509.Certificate {
-	if len(ctx.VerifiedChains) == 0 || len(ctx.VerifiedChains[0]) == 0 {
+	if ctx == nil || len(ctx.VerifiedChains) == 0 || len(ctx.VerifiedChains[0]) == 0 {
 		return nil
 	}