during EXPLAIN

Before the patch, send_eof() of some subclasses of 
select_result (e.g., select_send::send_eof()) could 
handle being called after an error had occured while others 
could not. The methods that were not well-behaved would trigger
an ASSERT on debug builds. Release builds were not affected.

Consider the following query as an example for how the ASSERT
could be triggered:

A user without execute privilege on f() does
   SELECT MAX(key1) INTO @dummy FROM t1 WHERE f() < 1;
resulting in "ERROR 42000: execute command denied to user..." 

The server would end the query by calling send_eof(). The 
fact that the error had occured would make the ASSERT trigger. 

select_dumpvar::send_eof() was the offending method in the
bug report, but the problem also applied to other 
subclasses of select_result. This patch uniforms send_eof() 
of all subclasses of select_result to handle being called 
after an error has occured.