net/portmapper: avert a panic when a mapping is not available (#16686)

Ideally when we attempt to create a new port mapping, we should not return
without error when no mapping is available. We already log these cases as
unexpected, so this change is just to avoiding panicking dispatch on the
invalid result in those cases. We still separately need to fix the underlying
control flow.

Updates #16662

Change-Id: I51e8a116b922b49eda45e31cd27f6b89dd51abc8

Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

net/portmapper/portmapper.go

@@ -507,6 +507,13 @@ func (c *Client) createMapping() {
 			c.logf("createOrGetMapping: %v", err)
 		}
 		return
+	} else if mapping == nil {
+		return
+
+		// TODO(creachadair): This was already logged in createOrGetMapping.
+		// It really should not happen at all, but we will need to untangle
+		// the control flow to eliminate that possibility. Meanwhile, this
+		// mitigates a panic downstream, cf. #16662.
 	}
 	if c.updates != nil {
 		c.updates.Publish(Mapping{