Do the safety check for the default coordinate index in CartesianPlot::load() before it's used in the children during the load

Description

Do the safety check for the default coordinate index in CartesianPlot::load() directly after the index was read from the XML and before it's being used in the children that are read subsequently.

This fixes the problem in #869 (closed). The actual root cause for the wrong index in the XML file remains to be understood, though.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #869 (closed)

When external dependencies are removed

• Reporting to ...