Exploit the fact that Expr.pRight and Expr.x are never used at the same time
for a small performance gain.

FossilOrigin-Name: aacbb9a461fdb34c7f9c8ce348e44c3e96c93334f210d438d92bfac1794dc686
diff --git a/src/expr.c b/src/expr.c
index 2e8b196..2841628 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1025,8 +1025,9 @@
     /* The Expr.x union is never used at the same time as Expr.pRight */
     assert( p->x.pList==0 || p->pRight==0 );
     if( p->pLeft && p->op!=TK_SELECT_COLUMN ) sqlite3ExprDeleteNN(db, p->pLeft);
-    sqlite3ExprDelete(db, p->pRight);
-    if( ExprHasProperty(p, EP_xIsSelect) ){
+    if( p->pRight ){
+      sqlite3ExprDeleteNN(db, p->pRight);
+    }else if( ExprHasProperty(p, EP_xIsSelect) ){
       sqlite3SelectDelete(db, p->x.pSelect);
     }else{
       sqlite3ExprListDelete(db, p->x.pList);