[Tile-serving] [openstreetmap/osm2pgsql] valgrind: get_sparse - move depends on uninitialised value (#1442)

[mmd]

Since @joto asked me to provide a fix, rather than just reporting random valgrind issues, my proposal would be an additional check in get_sparse to check for sizeSparseTuples == 0. This eliminates the valgrind error, while test cases are still passing.

Does this make sense?

```
osmium::Location node_ram_cache::get_sparse(osmid_t id) const
{
    if (sizeSparseTuples == 0) {
      return osmium::Location{};
    }

    int64_t pivotPos = sizeSparseTuples >> 1;
    int64_t minPos = 0;
    int64_t maxPos = sizeSparseTuples;

    while (minPos <= maxPos) {
```

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