Fix unsigned int overflow

Ghost User requested to merge (removed):patch-1 into master

When you call _xmlSchemaDateAdd() with a date like 2019-01-01+01:00 the function returns 4294969314-12-31 instead of 2018-12-31.

This fixes a relevant bug I reported last year on the old tracker: Bug 795432 - Schemas parser error when restricting date or dateTime with min and max values.

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