Fix assert() statements that would (incorrectly) fire if an IF NOT EXISTS

trigger that already exists contained two or more RETURNING clauses.
Tickets [89d259d45b855a0d] and [d15b3a4ea901ef0d].

FossilOrigin-Name: 648899e4ded72cac6cc24bccf7ebfc709ee7309a003452b21fd6ab0ba20c34b8

src/trigger.c

@@ -202,6 +202,7 @@ void sqlite3BeginTrigger(
       }else{
         assert( !db->init.busy );
         sqlite3CodeVerifySchema(pParse, iDb);
+        VVA_ONLY( pParse->ifNotExists = 1; )
       }
       goto trigger_cleanup;
     }