Fix core dump in an endcase of b.strip() that I missed.

Lib/test/test_bytes.py

@@ -653,6 +653,7 @@ class BytesTest(unittest.TestCase):
         self.assertEqual(b.strip(b'pi'), b'mississ')
         self.assertEqual(b.strip(b'im'), b'ssissipp')
         self.assertEqual(b.strip(b'pim'), b'ssiss')
+        self.assertEqual(b.strip(b), b'')
 
     def test_lstrip(self):
         b = b'mississippi'

Objects/bytesobject.c

@@ -2502,7 +2502,10 @@ bytes_strip(PyBytesObject *self, PyObject *arg)
     argptr = ((PyBytesObject *)arg)->ob_bytes;
     argsize = Py_Size(arg);
     left = lstrip_helper(myptr, mysize, argptr, argsize);
-    right = rstrip_helper(myptr, mysize, argptr, argsize);
+    if (left == mysize)
+        right = left;
+    else
+        right = rstrip_helper(myptr, mysize, argptr, argsize);
     return PyBytes_FromStringAndSize(self->ob_bytes + left, right - left);
 }