Merge two assertions to make comment clearer

Authored by Tom Lane, after a gripe from James Coleman.

Discussion: https://postgr.es/m/CAAaqYe9GD__4Crm=ddz+-XXcNhfY_V5gFYdLdmkFNq=2VHO56Q@mail.gmail.com

src/backend/utils/sort/tuplesort.c

@@ -1186,8 +1186,8 @@ void
 tuplesort_set_bound(Tuplesortstate *state, int64 bound)
 {
 	/* Assert we're called before loading any tuples */
-	Assert(state->status == TSS_INITIAL);
-	Assert(state->memtupcount == 0);
+	Assert(state->status == TSS_INITIAL && state->memtupcount == 0);
+
 	Assert(!state->bounded);
 	Assert(!WORKER(state));