[bnb] Check that some issues (Bände) are selected

[zuphilip]

Currently, it is possible to start the "Recherche" in recherche.php without choosing any issues. This leads to some errors/warnings in the apache log:

[Tue Feb 09 20:42:20.497881 2016] [:error] [pid xxx] [client xxx] PHP Notice:  Undefined variable: nrArray in /var/www/html/malibu/bnb/bnb.php on line 76, referer: http://data.bib.uni-mannheim.de/malibu/bnb/recherche.php?ddcgruppe%5B%5D=000%2C001%2C002%2C003
[Tue Feb 09 20:42:20.498118 2016] [:error] [pid xxx] [client xxx] PHP Warning:  Invalid argument supplied for foreach() in /var/www/html/malibu/bnb/bnb.php on line 76, referer: http://data.bib.uni-mannheim.de/malibu/bnb/recherche.php?ddcgruppe%5B%5D=000%2C001%2C002%2C003

Is it possible to check in JavaScript in recherche.php that at least one option is chosen? Moreover, it might be a good idea to restrict also the number of options by some limit, e.g. at most 4 issues can be selected.

Reported first in #32 (comment)

[zuphilip]

Moreover, we should restrict the number of issues selected, because each issue contains several MB which can sum up to some larger number, such that the server will reset the connection at some point. Maybe, let us say it should be between 1 and 10?