connectfour/controller: Prevent list access by negative indexing.

Currently, if a user does 'move 0', he moves to column 6(last column)
in connect_four which is unwanted behaviour and happening due to
list getting accessed through negative index (-1).

Added a check for that in controller.py file.

Fixes #343

zulip_bots/zulip_bots/bots/connect_four/connect_four.py

@@ -47,7 +47,7 @@ class ConnectFourBotHandler(GameAdapter):
         bot_name = 'connect_four'
         move_help_message = '* To make your move during a game, type\n' \
                             '```move <column-number>``` or ```<column-number>```'
-        move_regex = '(move (\d)$)|((\d)$)'
+        move_regex = '(move ([1-7])$)|(([1-7])$)'
         model = ConnectFourModel
         gameMessageHandler = ConnectFourMessageHandler