Make `expt` work again for inexact exponent + negative base

[jpellegrini]

stklos>  (expt -2.0 0.5)
-nan.0
stklos>  (expt -2.0 1/2)
8.65956056235493e-17+1.41421356237309i

Oops. This was my recent PR.

We should not call pow for negative base and inexact exponent.

[egallesio]

Merged. Thanks @jpellegrini