XdsClient: drop refs to ChannelState objects upon XdsClient shutdown (#30266)
XdsClient: drop refs to ChannelState objects upon XdsClient shutdown (#30266)

* XdsClient: drop refs to ChannelState objects upon XdsClient shutdown

* be defensive to make sure we don't access a null pointer
ctiller/grpc,ctiller/grpc,ctiller/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,ctiller/grpc,stanley-cheung/grpc,ctiller/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,grpc/grpc,ejona86/grpc,ejona86/grpc,jtattermusch/grpc,jtattermusch/grpc,ejona86/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,stanley-cheung/grpc,ejona86/grpc,ejona86/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,ejona86/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,ctiller/grpc,jtattermusch/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,ejona86/grpc,ctiller/grpc,ctiller/grpc,jtattermusch/grpc,jtattermusch/grpc,ejona86/grpc,ejona86/grpc,stanley-cheung/grpc,ctiller/grpc,grpc/grpc,grpc/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,stanley-cheung/grpc