extended_type_info_arg constructor calls base constructor with wrong argument

[kab@…]

In the file libs/serialization/src/extended_type_info.cpp, the constructor for the class extended_type_info_arg calls the base class constructor extended_type_info with an argument of NULL. The extended_type_info constructor takes an unsigned int argument, and NULL is not necessarily an unsigned int or even convertible to such. (See C.2.2.3 of C++98 standard.) Changing the argument to a constant 0 (or a named constant with that value) fixes the local problem, and doesn't conflict with either of the other existing classes derived from extended_type_info.

I noticed this problem in 1.38beta2, due to a warning on one of the platforms I'm using, but it looks like it's been this way "forever".