fix building wrong PROMPT_COMMAND

If one previously declared a perfectly valid `PROMPT_COMMAND` like 

    PROMPT_COMMAND="do_something;"

, *byobu* would have corrupted it: 

    PROMPT_COMMAND="$PROMPT_COMMAND;history -a;history -r"

resulted in

    "do_something;;history -a;history -r"

, but the double semicolons are illegal:

    bash: PROMPT_COMMAND: line 0: syntax error near unexpected token `;'
    bash: PROMPT_COMMAND: line 0: `do_something; ;history -a;history -r'