[vm/ffi] signed integer overflow in sample

[dcharkes]

https://dart-ci.appspot.com/log/vm-kernel-ubsan-linux-release-x64/dartk-ubsan-linux-release-x64/810/samples_2/ffi/samples_test

stderr:
../../runtime/bin/ffi_test/ffi_test_functions.cc:186:32: runtime error: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
    #0 0x7fdc3832213b in IntComputation ../../out/ReleaseUBSANX64/../../runtime/bin/ffi_test/ffi_test_functions.cc:186:32

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../runtime/bin/ffi_test/ffi_test_functions.cc:186:32 in

--- Re-run this test:
python tools/test.py -n dartk-ubsan-linux-release-x64 samples_2/ffi/samples_test

sdk/samples/ffi/sample_ffi_functions.dart

Lines 102 to 103 in 302de78

	var mint = 0x7FFFFFFFFFFFFFFF; // 2 ^ 63 - 1
	result = intComputation(1, 1, 0, mint);

sdk/runtime/bin/ffi_test/ffi_test_functions.cc

Lines 183 to 189 in 302de78

	DART_EXPORT int64_t IntComputation(int8_t a, int16_t b, int32_t c, int64_t d) {
	std::cout << "IntComputation(" << static_cast<int>(a) << ", " << b << ", "
	<< c << ", " << d << ")\n";
	const int64_t retval = d - c + b - a;
	std::cout << "returning " << retval << "\n";
	return retval;
	}

0x7FFFFFFFFFFFFFFF - 0 + 1 overflows.