Python buffer objects can have negative strides.

tests/test_numpy_array.cpp

@@ -13,6 +13,7 @@
 #include <pybind11/stl.h>
 
 #include <cstdint>
+#include <vector>
 
 using arr = py::array;
 using arr_t = py::array_t<uint16_t, 0>;
@@ -294,4 +295,4 @@ test_initializer numpy_array([](py::module &m) {
         std::fill(a.mutable_data(), a.mutable_data() + a.size(), 42.);
         return a;
     });
-});
+});