Fix a typo on masking face id

Most of functionality should be moved to FcFreeTypeQueryAll()
for varfonts now though, if doing the same to FcFreeTypeQuery()
returns Null pattern because of inappropriate masking.

This might be not that big deal for varfonts support. but

Fixes fontconfig/fontconfig#162

Reported by Kevin Scott
3 jobs for issue/162 in 5 minutes and 40 seconds
Status Job ID Name Coverage
  Build
passed #365656
mingw-build

00:04:29

passed #365654
shared-build

00:05:40

passed #365655
static-build

00:04:26