tic doesn't read from STDIN and segfaults when adding terminfo to support italics in tmux
So this works for me, notice I edited use like @mtklr suggested:

# A screen-256color based TERMINFO that adds the escape sequences for italic.
# run to add to term db: tic tmux.terminfo
tmux|tmux terminal multiplexer,
  ritm=\E[23m, rmso=\E[27m, sitm=\E[3m, smso=\E[7m, Ms@,
  use=xterm, use=screen,

tmux-256color|tmux with 256 colors,
  use=xterm-256color, use=tmux,


Then tic -x tmux.terminfo

Shouldn't have errors

Then added to .tmux.conf:

set -g default-terminal "tmux-256color"
set -as terminal-overrides ',xterm*:sitm=\E[3m'