Fix a crash when running with FC_DEBUG=256

Fixes fontconfig/fontconfig#167
3 jobs for issue/167 in 7 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #393283
mingw-build

00:04:42

passed #393281
shared-build

00:05:39

passed #393282
static-build

00:07:10