Bug#19392 Rename Database: Crash if case change

Problem:
Renaming a database to itself crashed server.
It hapenned because of wrong DBUG_ASSERT.
Fix: removing wrong DBUG_ASSERT. Now it reports
a correct error message "database alreadt exists".

mysql-test/r/renamedb.result

@@ -27,3 +27,7 @@ a
 2
 3
 drop database testdb2;
+create database testdb1;
+rename database testdb1 to testdb1;
+ERROR HY000: Can't create database 'testdb1'; database exists
+drop database testdb1;

mysql-test/t/renamedb.test

@@ -16,3 +16,11 @@ select database();
 show tables;
 select a from t1 order by a;
 drop database testdb2;
+
+#
+# Bug#19392 Rename Database: Crash if case change
+#
+create database testdb1;
+--error 1007
+rename database testdb1 to testdb1;
+drop database testdb1;

sql/sql_db.cc

@@ -134,8 +134,8 @@ void lock_db_delete(const char *name, uint length)
 {
   my_dblock_t *opt;
   safe_mutex_assert_owner(&LOCK_lock_db);
-  opt= (my_dblock_t *)hash_search(&lock_db_cache, (const byte*) name, length);
-  DBUG_ASSERT(opt != NULL);
+  if (opt= (my_dblock_t *)hash_search(&lock_db_cache,
+                                      (const byte*) name, length))
     hash_delete(&lock_db_cache, (byte*) opt);
 }