Fix an assert() in memdbTruncate() that could fail when processing a corrupt database.

FossilOrigin-Name: b1e2929860557cf88f98f0a4f2472e1a16be126bbb8050f0d728350f0cfe987a

test/memdb1.test

@@ -229,4 +229,40 @@ ifcapable vtab {
   } {1 {table t1 already exists}}
 }
 
+
+#-------------------------------------------------------------------------
+# dbsqlfuzz  0a13dfb474d4f2f11a48a2ea57075c96fb456dd7
+#
+reset_db
+do_execsql_test 800 {
+  PRAGMA autovacuum = 0;
+  PRAGMA page_size = 8192;
+  PRAGMA journal_mode = wal;
+  CREATE TABLE t1(x, y);
+  INSERT INTO t1 VALUES(1, 2);
+  CREATE TABLE t2(x, y);
+} {wal}
+db close
+
+set fd [open test.db]
+fconfigure $fd -translation binary -encoding binary
+set data [read $fd [expr 20*1024]]
+
+sqlite3 db ""
+db deserialize $data
+
+do_execsql_test 810 {
+  PRAGMA locking_mode = exclusive;
+  SELECT * FROM t1
+} {exclusive 1 2}
+
+do_execsql_test 820 {
+  INSERT INTO t1 VALUES(3, 4);
+  SELECT * FROM t1;
+} {1 2 3 4}
+
+do_catchsql_test 830 {
+  PRAGMA wal_checkpoint;
+} {1 {database disk image is malformed}}
+
 finish_test