Fix std::iscntrl use in gtest-printers.cc

ContainsUnprintableControlCodes() in gtest-printers.cc passes a char
argument to std::iscntrl. Although its argument is an int, std::iscntrl
produces undefined behavior if its argument is not representable as an
unsigned char. The standard library on Windows asserts that the argument
is an unsigned char, resulting in an assertion crash on debug builds.