THistSvc: fix memory corruption in deReg(string)

Deregistering a histogram via its name causes a memory corruption (often resulting in a crash). The problem was that deReg(string) kept checking the histogram list size while the list is actually being deleted in the last call to deReg(TObject*).

Also improved the unit test coverage for this case.

cc @leggett @merrenst

@leggett this one is quite urgent to get into ATLAS as it causes failures in most of our trigger tests in master: https://its.cern.ch/jira/browse/ATR-17887

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mentioned in merge request atlas/Gaudi!181 (merged)

approved this merge request

merged

mentioned in commit 3f991a2e