Remove rabit dependency on public headers. (#6005)

tests/cpp/common/test_span.cc

@@ -519,5 +519,12 @@ TEST(Span, Empty) {
   }
 }
 
+TEST(SpanDeathTest, Empty) {
+  std::vector<float> data(1, 0);
+  ASSERT_TRUE(data.data());
+  Span<float> s{data.data(), Span<float>::index_type(0)};  // ok to define 0 size span.
+  EXPECT_DEATH(s[0], "\\[xgboost\\] Condition .* failed.\n");  // not ok to use it.
+}
+
 }  // namespace common
 }  // namespace xgboost