gcd performs modulo by zero which is undefined

[James E. King, III]

Boost.Rational uses Boost.Integer's gcd method. I Added Coverity Scan support to the Boost.Rational build and it identified an issue that needs investigation:

CID 293013 (#4-12 of 12): Division or modulo by zero (DIVIDE_BY_ZERO) divide_by_zero: In function call gcd, modulo by expression 0 has undefined behavior. [hide details]

(rational_test.cpp) 470 BOOST_CHECK_EQUAL( boost::gcd<T>( 0, -9), static_cast<T>( 9) );

(rational.hpp) IntType gcd(IntType n, IntType m) {

Defer to the version in Boost.Integer

1. divide_arg: n is used as a divisor in function gcd.

return integer::gcd( n, m );

}