bash crashes if TERM is unset and the Delete key is pressed twice

[Emanuele Torre]

This happens since 88d69b4fa224d93ef1d26b80229668397bb6496b .

If bash is started with the TERM variable unset or empty, it will
segfault and crash if you press the Delete key twice (it only happens
for the first prompt, and if you don't press anything before the two
Delete key presses).

I was able to reproduce this in the linux virtual console, urxvt, and
xterm, both running tmux and not running tmux.

Reproduce with either of these commands:

    * env -i /path/to/bash-devel/bash --norc --noprofile
    * TERM= /path/to/bash-devel/bash --norc --noprofile

If it is not reproducible with your terminal emulator, try:

    * xterm -e env -i /path/to/bash-devel/bash --norc --noprofile
    * xterm -e env 'TERM=' /path/to/bash-devel/bash --norc --noprofile

 emanuele6