The a-real / b-non-real branch of Quo divided the real numerator by each
component of b independently, which is not complex division: (1+0i)/(1+i)
returned 1+1i instead of 0.5-0.5i, (1+0i)/i returned +Inf+1i, and (0+0i)/i
divided 0/0 and panicked. Use the full a*conj(b)/|b|^2 formula (the same
one the general branch uses), which reduces correctly when a is real.
Closes#577