Attempt to access TABLE instance when open table (to obtain
statistics) fails because of KILL query or connection operation
resulted in the issue reported.

For the non-innodb tables, its statistics is obtained by opening
the table. Current code assumes that when open table fails then
Diagnostics_area is always set with an error. If Diagnostics_area
is not set then open table success condition code is executed.
This works well when table open fails because of the error. Open
table fails even when query or connections is killed. In this case
Diagnostics_area might not be set. In such scenario, attempt
access TABLE instance from the pointer in TABLE_LIST resulted in
the crash.

To fix the issue, the failures from open_tables() is handled
irrespective of whether the diagnostics area is set. If we find
error being reported in diagnostics area, we write it to comment
field as we do now. If we do not find a error reported, then we
just make the UDF return failure. The query is later terminated
by call to send_kill_message() when we check thd->killed flag.