[gpkg] Fix adding field comments after alternative name

[nyalldawson]

Fix constraints on gpkg_data_columns are violated when AlterFieldDefn is called in some circumstances, eg when a comment is added after an alternative name has already been set on the field.

[coveralls]

coverage: 67.861% (+0.003%) from 67.858%
when pulling 8eea753 on nyalldawson:alter_comment_after_alt_name
into b2015f8 on OSGeo:master.

[nyalldawson]

Thanks @rouault , fixed

[rouault]

I've seen this is is likely related to qgis/QGIS#55304 . How does the GeoPackage driver reacts when having duplicated aliases: does it error out cleanly and return things in a consistent state?

[nyalldawson]

@rouault

I've seen this is is likely related to qgis/QGIS#55304

Not really related -- you can easily trigger the bug from raw gdal api by setting an alternative name, and then setting a comment in a second step. Eg from qgis if you set alias/comment via the GUI then it'll be two atomic operations like this, and the second will error out with the constraint violation.

How does the GeoPackage driver reacts when having duplicated aliases: does it error out cleanly and return things in a consistent state?

It errors out when trying to set the second field alias, with a message about the failed constraint. It's a relatively technical error message (it's the raw constraint violation message), but it DOES give sufficient information to see what's happening. Nothing is changed, so the GPKG remains valid.

[nyalldawson]

Thanks @rouault !