Fix a bug with attribute index handling in ModifyAttribute

This changes fixes a minor bug with proper attribute index range
checking in the server implementation of ModifyAttribute. A corner
case that previously slipped through index checking and testing
has been closed. An existing unit test has been modified to fix
this bug.

Partially implements #547

kmip/tests/unit/services/server/test_engine.py

@@ -4975,7 +4975,7 @@ class TestKmipEngine(testtools.TestCase):
                 attribute=attribute_factory.create_attribute(
                     enums.AttributeType.NAME,
                     "Modified Name",
-                    index=3
+                    index=1
                 )
             ),
         )