Reset errno to do error handling properly

This fixes the weird behavior when running with SOURCE_DATE_EPOCH=0:

Fontconfig: SOURCE_DATE_EPOCH: strtoull: No such file or directory: 0
3 jobs for reset-errno in 7 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #211365
mingw-build

00:07:23

passed #211363
shared-build

00:06:18

passed #211364
static-build

00:05:37