mdisplay returns an error "[: argument expected line 31

[xmailla]

Hello,

running mb and then showing a mail via #mailnumber returns an error.

It occurs to be at line 31:

[ $use_tmux -eq 0 -a -n $DISPLAY ] && use_xterm=1

Changing it to something like:

if [ -n $TMUX ]; then
      use_tmux=1
elif [ ${use_tmux} -eq 0 -a -n $DISPLAY ];then
       use_xterm=1
fi

fixes it here on my MacOS box (PR in approach)