If the value of factory's pax or mail demand overflows, no pax generated

[poppo]

When pax_level is very large, production level is large(about 9999), and city size is also large, pax&mail demand can overflow.

If the value overflows, pax or mail are not generated (because the value is treated as "not larger than 0").

[prissi]

Since such a big number are not really practical, I will cap the demand at 932,067 (2^23-1)/9. However, I fail to create these numbers in a test game. So please test the latest revision.

[poppo]

my test factory pax_level is 65535 and city population is about 99999.
I will check with latest version later.