message: ambulance: fix for
  The original test case relied on an invalidated iterator. This error
  did not occur before, but only with GCC with -D_GLIBCXX_DEBUG. This
  commit fixes the test case. The library is unaffected by this change.
diff: @@ -583,10 +583,11 @@ TEST_CASE("modifiers")
  SECTION("insert nothing (count = 0)")
  {
  - auto pos = j_array.end();
  auto it = j_array.insert(j_array.end(), 0, 5);
  CHECK(j_array.size() == 4);
  - CHECK(it == pos);
  + // the returned iterator points to the first inserted element;
  + // there were 4 elements, so it should point to the 5th
  + CHECK(it == j_array.begin() + 4);
  CHECK(j_array == json({1, 2, 3, 4}));
  }
  }
  