mysqld crashed when a long-running explain query was killed from
another connection.

When the current thread caught a kill signal executing the function
best_extension_by_limited_search it just silently returned to  
the calling function greedy_search without initializing elements of
the join->best_positions array.
However, the greedy_search function ignored thd->killed status
after a calls to the best_extension_by_limited_search function, and
after several calls the greedy_search function used an uninitialized
data from the join->best_positions[idx] to search position in the
join->best_ref array. 
That search failed, and greedy_search tried to call swap_variables
function with NULL argument - that caused a crash.