Fix error message formatting (possible SEGV) for prepare_transaction

If SESSION->prepare_transaction is called with a prepare_timestamp less than the previously set oldest timestamp, an error is returned (expected), but the error message does not fill the correct buffer with the oldest timestamp, so an uninitialized pointer is used in the message.

This was noticed with Python3 testing of test_prepare05.py and test_prepare06.py, they give a SEGV.  Python2 is apparently kinder, and some garbage appears as part of the error message, which is not noticed by the test.