system_error::what() doesn't incorporate code.message() if this->code().value() == 0

[Dmitry Goncharov <dgoncharov@…>]

if error_code is given 0 as the first argument then system_error::what() ignores code().message().

This piece of code

boost::system::error_code const ec(0, boost::system::system_category);
boost::system::system_error se(ec);
cout << se.what() << endl;

will not print anything. "Success" is expected.

[Dmitry Goncharov <dgoncharov@…>]

This patch fixes the bug. Tested on linux with gcc-4.4.2 and freebsd with gcc-4.2.1

[Beman Dawes]

(In [59673]) Fix #3927