Initialize Function::domain

Fixes uninitialized memory read in oss-fuzz 15395
6 jobs for fix_oss_fuzz in 4 minutes and 14 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #370308
build

00:03:25

passed #370313
build_android

00:01:04

passed #370309
build_clang_libcpp

00:03:17

passed #370312
build_clazy

00:03:15

passed #370311
build_mingw64_fedora

00:03:08

passed #370310
build_ubuntu_16_04

00:01:47