Flag unused pszLocale in CPLPrintDouble (#5414)
Cleanup CPLPrintDouble:
- Check pszBuffer before doing anything else
- Convert #define to const to make visible in debugger
- Move int i into the for loop


git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@28141 f0d54148-0727-0410-94bb-9a71ac55c965