Add an assert() as an additional test for fix in check-in [523b42371122d9e1b3].

FossilOrigin-Name: f9e85e6c79162081f8f512d22951931a89d157a32b24afdc48b7eb865379abe7

src/select.c

@@ -2540,6 +2540,7 @@ static int multiSelect(
   */
   assert( p && p->pPrior );  /* Calling function guarantees this much */
   assert( (p->selFlags & SF_Recursive)==0 || p->op==TK_ALL || p->op==TK_UNION );
+  assert( p->selFlags & SF_Compound );
   db = pParse->db;
   pPrior = p->pPrior;
   dest = *pDest;