LP:776285 - Crash when updating multiple equality const-ness during constant table optimization in make_join_statistics

The following test case extracted from the test for LP BUG#715759
results in a crash (NULL pointer):

CREATE TABLE t1 (a1 int, a2 int) ;
INSERT INTO t1 VALUES (1, 2);
INSERT INTO t1 VALUES (3, 4);

CREATE TABLE t2 (b1 int, b2 int) ;
INSERT INTO t2 VALUES (1, 2);

SELECT b1 FROM t2 WHERE b1 = b2;