Check number of flowers in game constructor

There's a check in the command line parser that makes sure that the
number of flowers is at most the number of tiles. It may at some point
become possible to change the default values even after that. The check
should be moved so that it runs even if the default values changes. The
FlowersGame constructor is probably a good candidate. The number of
flowers given to it is automatically changed if necessary.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>