Function MAKE_DBKEY may produce random errors if used with relation name

[aafemt]

Because function LiteralNode::getText() returns string which is not zero-terminated. Particularly QA test gh_8168_test.py sometimes unexpectedly produces "Malformed string" error.

[hvlad]

Few notes:

• it happens in DEBUG build with table names of any length and in RELEASE build with table names greater than 15 bytes.
  As QA test gh_8168_test uses short table names, it could fail with DEBUG build only
• gbak is not affected as it uses table ID's, not names.