KILL_BAD_DATA is returned

Two problems discovered with the LEAST()/GREATEST() 
functions:
1. The check for a null value should happen even 
after the second call to val_str() in the args. This is
important because two subsequent calls to the same
Item::val_str() may yield different results.
Fixed by checking for NULL value before dereferencing
the string result.

2. While looping over the arguments and evaluating them 
the loop should stop if there was an error evaluating so far
or the statement was killed. Fixed by checking for error
and bailing out.