config: drop cf validity check in get_next_char()

config: drop cf validity check in get_next_char()

The global variable cf is set with an initialized value in all codepaths before
calling this function.

The complete call graph looks like this:

  git_config_from_file
    -> do_config_from
      -> git_parse_file
        -> get_next_char
        -> get_value
            -> get_next_char
            -> parse_value
                -> get_next_char
        -> get_base_var
            -> get_next_char
            -> get_extended_base_var
                -> get_next_char

The variable is initialized in do_config_from.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>