fixed length of current database name if it is dropped (BUG#1986)

mysql-test/r/query_cache.result:
  test of BUG#1986
mysql-test/t/query_cache.test:
  test of BUG#1986
sql/sql_db.cc:
  as far as we remove db name we should remove its length

mysql-test/r/query_cache.result

@@ -809,4 +809,12 @@ show status like "Qcache_queries_in_cache";
 Variable_name	Value
 Qcache_queries_in_cache	4
 DROP TABLE t1;
+CREATE TABLE t1 (a int(1));
+CREATE DATABASE mysqltest;
+USE mysqltest;
+DROP DATABASE mysqltest;
+SELECT * FROM test.t1;
+a
+USE test;
+DROP TABLE t1;
 SET GLOBAL query_cache_size=0;

mysql-test/t/query_cache.test

@@ -585,4 +585,17 @@ show status like "Qcache_queries_in_cache";
 # Keep things tidy
 #
 DROP TABLE t1;
+
+#
+# DROP current database test
+#
+CREATE TABLE t1 (a int(1));
+CREATE DATABASE mysqltest;
+USE mysqltest;
+DROP DATABASE mysqltest;
+SELECT * FROM test.t1;
+USE test;
+DROP TABLE t1;
+
+
 SET GLOBAL query_cache_size=0;

sql/sql_db.cc

@@ -409,7 +409,7 @@ exit:
     when the slave is replicating a DROP DATABASE: 
     - garbage characters in the error message:
     "Error 'Can't drop database 'test2'; database doesn't exist' on query
-    'h4zI'"
+    'h4zI'"
     - segfault
     - hang in "free(vio)" (yes!) in the I/O or SQL slave threads (so slave
     server hangs at shutdown etc).
@@ -419,6 +419,7 @@ exit:
     if (!(thd->slave_thread)) /* a slave thread will free it itself */
       x_free(thd->db);
     thd->db= 0;
+    thd->db_length= 0;
   }
 exit2:
   VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));