Fixing a logic error in THistSvc::deReg that makes it impossible to de-register histograms

I really don't understand why, but we only started seeing issues in the ATLAS monitoring code in the last few weeks. Issues with de-registering histograms.

After some debugging, I found that this if statement was set up incorrectly. The error condition is when the iterator that we're trying to find is the end iterator. Not when it's something else.

With this fix the ATLAS code seems to behave as expected.