Fix NULL pointer dereference in tuplesort.c.

Oversight in commit e94568ec.  This could cause a crash when an external
datum tuplesort of a pass-by-value type required multiple passes.
Per report from Mithun Cy.

Peter Geoghegan

Discussion: https://postgr.es/m/CAD__OujuhfWFULGFSt1fyHqUb8N-XafjJhudwt88V0Qs2o84qg@mail.gmail.com