Fixes to a couple of large-ish memory leaks

A few GPU memory leaks were reported, and the fixes confirmed, by running the compute-sanitizer on a celeritas test:

build> compute-sanitizer --print-limit 120 --leak-check full test/celeritas_geo_Geometry --gtest_filter=TestEm3*

Output before fixes:

========= LEAK SUMMARY: 19584 bytes leaked in 106 allocations
========= ERROR SUMMARY: 106 errors

Output after fixes:

========= LEAK SUMMARY: 832 bytes leaked in 103 allocations
========= ERROR SUMMARY: 103 errors