such as quit and shutdown

Logging to slow log can produce an undetermined value for
Rows_examined in special cases. In debug mode this manifests
itself as any of the various marker values used to mark
uninitialized memory on various platforms.

If logging happens on a THD object that hasn't performed any
row reads (on this or any previous connections), the
THD::examined_row_count may be uninitialized. This patch adds
initialization for this attribute.

No automated test cases are added, as for this to be
meaningful, we need to ensure that we're using a THD
fulfilling the above conditions. This is hard to do in the
mysql-test-run framework. The patch has been verified
manually, however, by restarting mysqld and running the test
included with the bug report.