Fix buglet in slice assignment of bytesobjects: assigning to b[3:0] ('stop'

being before 'start') would actually assign to b[0:0] (or whatever 'stop'
was)

Lib/test/test_bytes.py

@@ -236,6 +236,9 @@ class BytesTest(unittest.TestCase):
         b[3:5] = [3, 4, 5, 6]
         self.assertEqual(b, bytes(range(10)))
         
+        b[3:0] = [42, 42, 42]
+        self.assertEqual(b, bytes([0, 1, 2, 42, 42, 42, 3, 4, 5, 6, 7, 8, 9]))
+
     def test_setslice_trap(self):
         # This test verifies that we correctly handle assigning self
         # to a slice of self (the old Lambert Meertens trap).

Objects/bytesobject.c

@@ -310,6 +310,8 @@ bytes_setslice(PyBytesObject *self, Py_ssize_t lo, Py_ssize_t hi,
 
     if (lo < 0)
         lo = 0;
+    if (hi < lo)
+        hi = lo;
     if (hi > self->ob_size)
         hi = self->ob_size;