FROM_PY_FUNCTION: Cast to sdigit after unary minus, not before

The current cast isn't doing anything, because the unary minus implicitly
upcasts to int again, resulting in warnings about potentially value altering
conversions on 32 bit systems.

Fixes issue #877.

Cython/Utility/TypeConversion.c

@@ -658,7 +658,7 @@ static CYTHON_INLINE {{TYPE}} {{FROM_PY_FUNCTION}}(PyObject *x) {
             const digit* digits = ((PyLongObject*)x)->ob_digit;
             switch (Py_SIZE(x)) {
                 case  0: return ({{TYPE}}) 0;
-                case -1: __PYX_VERIFY_RETURN_INT({{TYPE}}, sdigit, -(sdigit) digits[0])
+                case -1: __PYX_VERIFY_RETURN_INT({{TYPE}}, sdigit, (sdigit) -digits[0])
                 case  1: __PYX_VERIFY_RETURN_INT({{TYPE}},  digit, +digits[0])
                 {{for _size in (2, 3, 4)}}
                 {{for _case in (-_size, _size)}}