Added a test case for UNION, verified it crashed before fix.
Added a test case for UNION, verified it crashed before fix.

LGTM Sugu.

nso95/vitess,nso95/vitess,fanngyuan/vitess,nso95/vitess,fanngyuan/vitess,fanngyuan/vitess,nso95/vitess,nso95/vitess,fanngyuan/vitess,fanngyuan/vitess