Add some const

Suggested by clang-tidy readability-non-const-parameter check
6 jobs for readability-non-const-parameter in 3 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #672759
build

00:03:30

passed #672764
build_android

00:01:55

passed #672760
build_clang_libcpp

00:02:48

passed #672763
build_clazy

00:03:28

passed #672762
build_mingw64_fedora

00:03:35

passed #672761
build_ubuntu_16_04

00:02:20