SF bug 533198: Complex power underflow raises exception.

Konrad was too kind.  Not only did it raise an exception, the specific
exception it raised made no sense.  These are old bugs in complex_pow()
and friends:

1. Raising 0 to a negative power isn't a range error, it's a domain
   error, so changed c_pow() to set errno to EDOM in that case instead
   of ERANGE.

2. Changed complex_pow() to:

A. Used the Py_ADJUST_ERANGE2 macro to try to clear errno of a spurious
   ERANGE error due to underflow in the libm pow() called by c_pow().

B. Produced different exceptions depending on the errno value:
   i) For errno==EDOM, raise ZeroDivisionError instead of ValueError.
      This is for consistency with the non-complex cases 0.0**-2 and
      0**-2 and 0L**-2.
   ii) For errno==ERANGE, raise OverflowError.

Bugfix candidate.

Objects/complexobject.c

@@ -131,7 +131,7 @@ c_pow(Py_complex a, Py_complex b)
 	}
 	else if (a.real == 0. && a.imag == 0.) {
 		if (b.imag != 0. || b.real < 0.)
-			errno = ERANGE;
+			errno = EDOM;
 		r.real = 0.;
 		r.imag = 0.;
 	}
@@ -456,11 +456,17 @@ complex_pow(PyComplexObject *v, PyObject *w, PyComplexObject *z)
 		p = c_pow(v->cval,exponent);
 
 	PyFPE_END_PROTECT(p)
-	if (errno == ERANGE) {
-		PyErr_SetString(PyExc_ValueError,
+	Py_ADJUST_ERANGE2(p.real, p.imag);
+	if (errno == EDOM) {
+		PyErr_SetString(PyExc_ZeroDivisionError,
 				"0.0 to a negative or complex power");
 		return NULL;
 	}
+	else if (errno == ERANGE) {
+		PyErr_SetString(PyExc_OverflowError,
+				"complex exponentiaion");
+		return NULL;
+	}
 	return PyComplex_FromCComplex(p);
 }