Fix how strides is set on Py_buffer object when the array is C-contiguous

This is a possible fix for #5085.

A complete solution would probably convert the strides array from something containing -1's into the equivalent without them, but I don't know all the corner cases involved with that.
make this:
```
if (PyArray_CHKFLAGS(self, NPY_ARRAY_C_CONTIGUOUS) &&
        !((flags & PyBUF_F_CONTIGUOUS) == PyBUF_F_CONTIGUOUS) {
```
making a test for this might be  PITA, though.