Fix UB in DetectorIDTest.cxx

[tvami]

I am updating ldmx-sw, here are the details.

What are the issues that this addresses?

This is the last bit of #1166

Check List

• I successfully compiled ldmx-sw with my developments
• I ran my developments and the following shows that they are successful.

I ran

denv cmake -B build -S . -DENABLE_SANITIZER_UNDEFINED_BEHAVIOR=ON
denv cmake --build build --target install -- -j$(nproc)
denv build/run_test [DetDescr]

which shows the problem before this fix, now it's gone.

[vamitamas@pod-login1 ldmx-sw]$ denv build/run_test [DetDescr]
Filters: [DetDescr]
Randomness seeded to: 1942624396
===============================================================================
All tests passed (92 assertions in 1 test case)


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