tso: pd nit pointer panic in tso dispatcher

[bufferflies]

Bug Report

What did you do?

What did you expect to see?

normal

What did you see instead?

panic

2023-03-09T10:05:14.8268865Z [2023/03/09 10:05:13.501 +00:00] [INFO] [client.go:290] ["[pd] create pd client with endpoints"] [pd-address="[https://127.0.0.1:36315]"]
2023-03-09T10:05:14.8269328Z [2023/03/09 10:05:13.554 +00:00] [INFO] [base_client.go:480] ["[pd] switch leader"] [new-leader=https://127.0.0.1:36315] [old-leader=]
2023-03-09T10:05:14.8269675Z [2023/03/09 10:05:13.554 +00:00] [INFO] [base_client.go:156] ["[pd] init cluster id"] [cluster-id=7208485466879229826]
2023-03-09T10:05:14.8270049Z [2023/03/09 10:05:13.555 +00:00] [INFO] [tso_request_dispatcher.go:287] ["[pd/tso] tso dispatcher created"] [dc-location=global]
2023-03-09T10:05:14.8270446Z [2023/03/09 10:05:13.555 +00:00] [INFO] [client.go:290] ["[pd] create pd client with endpoints"] [pd-address="[https://127.0.0.1:36315]"]
2023-03-09T10:05:14.8270786Z [2023/03/09 10:05:13.562 +00:00] [ERROR] [tso_request_dispatcher.go:507] ["[pd/tso] update connection contexts failed"] [dc=global] [error="rpc error: code = Canceled desc = context canceled"]
2023-03-09T10:05:14.8271151Z [2023/03/09 10:05:13.562 +00:00] [INFO] [tso_request_dispatcher.go:305] ["[pd/tso] exit tso dispatcher"] [dc-location=global]
2023-03-09T10:05:14.8271355Z panic: runtime error: invalid memory address or nil pointer dereference
2023-03-09T10:05:14.8271548Z [signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x1eedf59]
2023-03-09T10:05:14.8271566Z 
2023-03-09T10:05:14.8271675Z goroutine 348500 [running]:
2023-03-09T10:05:14.8271933Z github.com/tikv/pd/client.(*client).tryResourceManagerConnect(0xc007054720?, {0x554f930, 0xc00a167bd0}, 0xc0057c1e58)
2023-03-09T10:05:14.8272146Z 	/home/runner/work/pd/pd/client/resource_manager_client.go:354 +0x119
2023-03-09T10:05:14.8272414Z github.com/tikv/pd/client.(*client).handleResourceTokenDispatcher(0xc015a9ae10, {0x554f930, 0xc00a167bd0}, 0xc015bdd300)
2023-03-09T10:05:14.8272628Z 	/home/runner/work/pd/pd/client/resource_manager_client.go:278 +0xdb
2023-03-09T10:05:14.8272820Z created by github.com/tikv/pd/client.(*client).createTokenDispatcher
2023-03-09T10:05:14.8273015Z 	/home/runner/work/pd/pd/client/resource_manager_client.go:265 +0x2cf
2023-03-09T10:05:14.8273162Z FAIL	github.com/tikv/pd/tests/client	230.444s
2023-03-09T10:05:14.8273243Z FAIL

What version of PD are you using (pd-server -V)?

master

[bufferflies]

/assign @JmPotato

[lhy1024]

cc @binshi-bing