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

src/backend/utils/sort/tuplesort.c

@@ -2800,6 +2800,7 @@ mergeonerun(Tuplesortstate *state)
 		WRITETUP(state, destTape, &state->memtuples[0]);
 
 		/* recycle the slot of the tuple we just wrote out, for the next read */
+		if (state->memtuples[0].tuple)
 			RELEASE_SLAB_SLOT(state, state->memtuples[0].tuple);
 
 		/*