FoFiTrueType::cvtSfnts: Fix uninitialized memory read on broken files

Initialize maxUsedGlyph after the early check returns

oss-fuzz/29629
7 jobs for maxUsedGlyph in 5 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #6694226
build

00:05:29

passed #6694231
build_android

00:00:51

passed #6694227
build_clang_libcpp

00:05:52

passed #6694230
build_clazy

00:04:49

passed #6694229
build_mingw64_fedora

00:03:20

passed #6694228
build_ubuntu_16_04

00:03:06

passed #6694225
clang_format

00:01:14