core/vdbe: Fix BEGIN after BEGIN CONCURRENT check

We're supposed to error out only when "is_begin" is true.

core/vdbe/execute.rs

@@ -2474,11 +2474,13 @@ pub fn op_auto_commit(
             }
         } else {
             let is_begin = !*auto_commit && !*rollback;
+            if is_begin {
                 return Err(LimboError::TxError(
                     "cannot use BEGIN after BEGIN CONCURRENT".to_string(),
                 ));
             }
         }
+    }
 
     program
         .commit_txn(pager.clone(), state, mv_store, *rollback)