Fix Ping upstream cause ctrld crash

dohTransport returns a http.RoundTripper. When pinging upstream, we do
it both for doh and doh3, and checking whether the transport is nil
before performing the check.

However, dohTransport returns a concrete *http.Transport. Thus
dohTransport will always return a non-nil http.Roundtripper, causing
invalid memory dereference when upstream is configured to use doh3.

Performing ping upstream separately will fix the issue.

config.go

@@ -407,8 +407,12 @@ func (uc *UpstreamConfig) Ping() {
 	}
 
 	for _, typ := range []uint16{dns.TypeA, dns.TypeAAAA} {
-		ping(uc.dohTransport(typ))
-		ping(uc.doh3Transport(typ))
+		switch uc.Type {
+		case ResolverTypeDOH:
+			ping(uc.dohTransport(typ))
+		case ResolverTypeDOH3:
+			ping(uc.doh3Transport(typ))
+		}
 	}
 }