Fix computation of outer-stride when calling .real() or .imag()
(transplanted from 4bfed5b6aa24003068192890c04b28d2278808db)

--HG--
branch : 3.1
extra : transplant_source : K%FE%D5%B6%AA%24%000h%19%28%90%C0K%28%D2%27%88%08%DB