avoid signed-integer underflow, which is undefined behavior

maybe these tests should just be deleted?

UBSAN gave:
runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long'
runtime error: signed integer overflow: -9223372036854775808 - 2 cannot be represented in type 'long'

test/unittest/valuetest.cpp

@@ -545,8 +545,10 @@ TEST(Value, Int64) {
     // Templated functions
     EXPECT_TRUE(z.Is<int64_t>());
     EXPECT_EQ(i, z.Get<int64_t>());
+#if 0 // signed integer underflow is undefined behaviour
     EXPECT_EQ(i - 1, z.Set(i - 1).Get<int64_t>());
     EXPECT_EQ(i - 2, z.Set<int64_t>(i - 2).Get<int64_t>());
+#endif
 }
 
 TEST(Value, Uint64) {