Fix name resolution problem in sub-selects within triggers, ticket [4ef7e3cfca].

FossilOrigin-Name: 5bcd0b1ca5d73ffbe7978ee9d73fe5e769e3d3a2
diff --git a/src/resolve.c b/src/resolve.c
index 07e0e91..86169c5 100644
--- a/src/resolve.c
+++ b/src/resolve.c
@@ -337,6 +337,8 @@
       }else if( op!=TK_INSERT && sqlite3StrICmp("old",zTab)==0 ){
         pExpr->iTable = 0;
         pTab = pParse->pTriggerTab;
+      }else{
+        pTab = 0;
       }
 
       if( pTab ){