Fix complex powers of negative real values.

Fixes GH issue #1538.

Cython/Utility/Complex.c

@@ -264,9 +264,13 @@ static {{type}} __Pyx_PyComplex_As_{{type_name}}(PyObject* o) {
             if (a.imag == 0) {
                 if (a.real == 0) {
                     return a;
-                }
+                } else if (a.real > 0) {
                     r = a.real;
                     theta = 0;
+                } else {
+                    r = -a.real;
+                    theta = atan2{{m}}(0, -1);
+                }
             } else {
                 r = __Pyx_c_abs{{func_suffix}}(a);
                 theta = atan2{{m}}(a.imag, a.real);

tests/run/complex_numbers_T305.pyx

@@ -48,6 +48,8 @@ def test_pow(double complex z, double complex w, tol=None):
     True
     >>> test_pow(complex(0.5, -.25), complex(3, 4), 1e-15)
     True
+    >>> test_pow(-0.5, 1j, tol=1e-15)
+    True
     """
     if tol is None:
         return z**w