fix: prevent NPE in RetriableStream when cancel() races start() (#12964) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Fix: Prevent NPE in RetriableStream when cancel() races start()

Bug: When cancel() is called through ClientCallStreamObserver (from application code in ClientResponseObserver.beforeStart()) before RetriableStream.start() completes, safeCloseMasterListener tries to dereference masterListener which is still null, causing an NPE.

Root cause: ClientCallImpl.startInternal assigns the stream at line 250 and starts it at line 285. RetriableStream.masterListener is only assigned as the first statement of start() (line 388). A cancel() arriving in that window passes the stream != null guard in cancelInternal but finds masterListener null.

Fix (two changes):

1. safeCloseMasterListener: Null-check masterListener before dereferencing. If masterListener is null (cancel before start), the close reason is already saved in savedCloseMasterListenerReason and start() will pick it up.

2. start(): After assigning masterListener, check if savedCloseMasterListenerReason is already set (from a prior cancel). If so, close immediately with the saved reason instead of proceeding with the normal start flow.

Fixes #12964

[linux-foundation-easycla]

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