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

Author: PeterHamilton

kmip/services/server/engine.py

@@ -1876,7 +1876,7 @@ def _process_modify_attribute(self, payload):
                     managed_object,
                     attribute_name
                 )
-                if 0 <= attribute_index <= len(existing_attributes):
+                if 0 <= attribute_index < len(existing_attributes):
                     self._set_attribute_on_managed_object_by_index(
                         managed_object,
                         attribute_name,

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

@@ -4975,7 +4975,7 @@ def test_modify_attribute_with_multivalued_no_index_match(self):
                 attribute=attribute_factory.create_attribute(
                     enums.AttributeType.NAME,
                     "Modified Name",
-                    index=3
+                    index=1
                 )
             ),
         )