Move second test on `ishape_last` inside the first

Move the second test on `self._ishape_last[i_slice][orientation]`
inside the first test. Avoid the double test on
`self._ishape_last[i_slice][orientation]` when
`self._ishape_last[i_slice][orientation]` is not None on entry of the
function. If `self._ishape_last[i_slice][orientation]` is not None on
entry of the function then, necessarily, the assertion on
`self._ishape_last[i_slice][orientation].size` and the definition of
`self._e_max[i_slice][orientation]` have already been done because
`self._ishape_last[i_slice][orientation]` and
`self._e_max[i_slice][orientation]` are only defined from the function
get_ishape_last.