pybind11_getbuffer: useless safe nullptr check (#1664)

Alternative implementation for #1657: if
we know that `obj` is never a `nullptr` [1], we should
not `nullptr`-check it *after* already dereferencing it.

[1] https://github.com/pybind/pybind11/pull/1657#issuecomment-452090058

include/pybind11/detail/class.h

@@ -469,7 +469,7 @@ extern "C" inline int pybind11_getbuffer(PyObject *obj, Py_buffer *view, int fla
         if (tinfo && tinfo->get_buffer)
             break;
     }
-    if (view == nullptr || obj == nullptr || !tinfo || !tinfo->get_buffer) {
+    if (view == nullptr || !tinfo || !tinfo->get_buffer) {
         if (view)
             view->obj = nullptr;
         PyErr_SetString(PyExc_BufferError, "pybind11_getbuffer(): Internal error");