Fix underflow/overflow in EcalVeto's closest cell calc

[tvami]

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

What are the issues that this addresses?

Part of the #1166

As for the source of the issue, the recoilPos is already outside the ECAL, and that's calculated at the SP. However the SP is bigger then the ECAL, so we can have hits that are on the ECAL SP, but are not on the real ECAL. This PR deals with those hits, in a reasonable way with the X position. But to be extra sure we dont reach the underflow and overflow in the mapsx/mapsy, I also edited the while loop

Check List

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

Compiling with ASAN build

ldmx cmake -B build -S . -DENABLE_SANITIZER_ADDRESS=ON
ldmx cmake --build build --target install -- -j$(nproc)
<run the ecal pn validation config with seed=1 and at event 142 you'll run into one issue>

the problem is gone.

[tomeichlersmith]

🧹 Thank you 🎉