StoreGate: Fix potential deadlock in removeProxy.

Scott Snyderrequested to merge
ssnyder/athena:remove.StoreGate-20201215 into master

In removeProxy(), don't try to get a pointer from a proxy if it's not valid. Doing so may trigger I/O. Besides being useless here, we may get deadlocks if we call into I/O code while holding the SG lock.

See ATEAM-685.

Merge request reports