slice2java: fix max parameter count for constructors

[externl]

Java constructors must have a "length" that is less than 255. This length is not the number of parameters, but a special UNIT where all types are size 1 except long and double, which are 2.

The slice2java constructor does not count this number correctly and only looks at the number of parameters.

https://github.com/zeroc-ice/ice/blob/3.7/cpp/src/slice2java/Gen.cpp#L2554-L2557

This means that we can generate code that does not compile. For instance if you have a class with 127 doubles.

Proposed fix:
Fix the check to account for the correct size of long and double.