pmtiles: always set leafStart when closing file [#794]

[bdon]

• the leafStart was only assigned in flushEntries, which never happens if there is only a root directory.
• resulted in erroneous leafDirectoryOffset and tileDataLength in the pmtiles header.

Detect with pmtiles verify:

pmtiles verify main.pmtiles
2025/01/30 15:11:57 main.go:223: Failed to verify archive, Leaf directories offset=0 must not be 0
exit status 1

Fixes #794

[systemed]

Thank you!