Issue #789290: make sure that hash(2**63) == hash(2.**63) on 64-bit

platforms.  The previous code was fragile, depending on the twin
accidents that:

  (1) in C, casting the double value 2.**63 to long returns the integer
      value -2**63, and
  (2) in Python, hash(-2**63) == hash(2**63).

There's already a test for this in test_hash.

Objects/object.c

@@ -1028,7 +1028,7 @@ _Py_HashDouble(double v)
 	fractpart = modf(v, &intpart);
 	if (fractpart == 0.0) {
 		/* This must return the same hash as an equal int or long. */
-		if (intpart > LONG_MAX || -intpart > LONG_MAX) {
+		if (intpart > LONG_MAX/2 || -intpart > LONG_MAX/2) {
 			/* Convert to long and use its hash. */
 			PyObject *plong;	/* converted to Python long */
 			if (Py_IS_INFINITY(intpart))