The problem query outer-joins a regular table with a derived table that
is an implicitly grouped table (it selects the row count from a join of
two inner tables). Before make_join_statistics() is called for the
outer query, the derived table is materialized. The derived table's join
is determined to have zero rows (due to the ON FALSE condition), and
this row count is propagated in join->best_rowcount.

In st_select_lex_unit::optimize(), this value is used to populate
result->estimated_rowcount, which make_join_statistics() later uses
to determine that the derived table is empty, and in case label
extract_empty_table, the table is extracted. However, due to the
derived table being implicitly grouped, one row is contained in the
table, and when join_read_const_table() is called for the table, a row
is found and the condition for it is evaluated. This condition refers to
table t1 which is not yet read, and a valgrind exception or other bad
things may happen.

The fix is to notice in st_select_lex_unit::optimize() that the row count
for an implicitly grouped table is exactly 1 (or, if HAVING is also
present, 0 or 1 rows).

mysql-test/r/derived.result
  Plan is changed from ref access to ALL because the row count is
  increased from 0 to 1.

mysql-test/r/heap.result
  Plan is changed from ref access to ALL because the row count is
  reduced from 2 to 1.

sql/sql_union.cc
  In st_select_lex_unit::optimize(), notice that the row count for
  an implicitly grouped query is (maximum) 1.