Fix a memory leak introduced in the previous commit.

[FreddieWitherden]

This fixes some memory leaks which my previous commit introduced.

ASAN is still showing one leak when running the tests:

Direct leak of 120 byte(s) in 1 object(s) allocated from:
    #0 0x7f8b634f9e68 in operator new(unsigned long) (/usr/lib/gcc/x86_64-pc-linux-gnu/14/libasan.so.8+0xf9e68)
    #1 0x7f8b631df4df in SpatialIndex::RTree::ExternalSorter::getNextRecord() /home/freddie/Programming/libspatialindex/src/rtree/BulkLoader.cc:297
    #2 0x7f8b631e0ad2 in SpatialIndex::RTree::BulkLoader::createLevel(SpatialIndex::RTree::RTree*, std::shared_ptr<SpatialIndex::RTree::ExternalSorter>, unsigned int, unsigned int, unsigned int, unsigned int, std::shared_ptr<SpatialIndex::RTree::ExternalSorter>, unsigned int, unsigned int) /home/freddie/Programming/libspatialindex/src/rtree/BulkLoader.cc:422
    #3 0x7f8b631dfee4 in SpatialIndex::RTree::BulkLoader::bulkLoadUsingSTR(SpatialIndex::RTree::RTree*, SpatialIndex::IDataStream&, unsigned int, unsigned int, unsigned int, unsigned int) /home/freddie/Programming/libspatialindex/src/rtree/BulkLoader.cc:354
    #4 0x7f8b6321d4e0 in SpatialIndex::RTree::createAndBulkLoadNewRTree(SpatialIndex::RTree::BulkLoadMethod, SpatialIndex::IDataStream&, SpatialIndex::IStorageManager&, double, unsigned int, unsigned int, unsigned int, SpatialIndex::RTree::RTreeVariant, long&) /home/freddie/Programming/libspatialindex/src/rtree/RTree.cc:211
    #5 0x557283599bc6 in main /home/freddie/Programming/libspatialindex/test/rtree/RTreeBulkLoad.cc:142
    #6 0x7f8b62a5142d  (/usr/lib64/libc.so.6+0x2642d)

SUMMARY: AddressSanitizer: 120 byte(s) leaked in 1 allocation(s).

but I believe this to be pre-existing.