Fixes bug #14569. When no db is selected as current and we do create procedure db.sp()...

we changing current db temporarily and restore it when sp is created. however thd->db
in this case becomes empty string rather than NULL and so all checks of thd->db == NULL 
will be false. So if after this we'll issue create procedure sp2()... without specifying
db it will succeed and create sp with db=NULL, which causes mysqldto crash on 
show procedure status statement.

This patch fixes the problem.

mysql-test/r/sp-error.result

@@ -916,3 +916,16 @@ ERROR 42S22: Unknown column 'bug13037_foo' in 'field list'
 DROP PROCEDURE bug13037_p1;
 DROP PROCEDURE bug13037_p2;
 DROP PROCEDURE bug13037_p3;
+create database mysqltest1;
+create database mysqltest2;
+use mysqltest1;
+drop database mysqltest1;
+create procedure mysqltest2.p1() select version();
+create procedure p2() select version();
+ERROR 3D000: No database selected
+use mysqltest2;
+show procedure status;
+Db	Name	Type	Definer	Modified	Created	Security_type	Comment
+mysqltest2	p1	PROCEDURE	root@localhost	0000-00-00 00:00:00	0000-00-00 00:00:00	DEFINER	
+drop database mysqltest2;
+use test;

mysql-test/t/sp-error.test

@@ -1337,6 +1337,22 @@ DROP PROCEDURE bug13037_p1;
 DROP PROCEDURE bug13037_p2;
 DROP PROCEDURE bug13037_p3;
 
+#
+# Bug#14569 "editing a stored procedure kills mysqld-nt"
+#
+create database mysqltest1;
+create database mysqltest2;
+use mysqltest1;
+drop database mysqltest1;
+create procedure mysqltest2.p1() select version();
+--error ER_NO_DB_ERROR 
+create procedure p2() select version();
+use mysqltest2;
+--replace_column 5 '0000-00-00 00:00:00' 6 '0000-00-00 00:00:00'
+show procedure status;
+drop database mysqltest2;
+use test;
+
 
 # BUG#NNNN: New bug synopsis
 #

sql/sql_db.cc

@@ -1163,8 +1163,17 @@ bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
   }
 end:
   x_free(thd->db);
-  thd->db=dbname;				// THD::~THD will free this
-  thd->db_length=db_length;
+  if (dbname && dbname[0] == 0)
+  {
+    x_free(dbname);
+    thd->db= NULL;
+    thd->db_length= 0;
+  }
+  else
+  {
+    thd->db= dbname;				// THD::~THD will free this
+    thd->db_length= db_length;
+  }
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
   if (!no_access_check)
     sctx->db_access= db_access;

sql/sql_parse.cc

@@ -4099,7 +4099,7 @@ end_with_restore_list:
 
     if (!lex->sphead->m_db.str || !lex->sphead->m_db.str[0])
     {
-      if (!thd->db || thd->db[0] == 0)
+      if (!thd->db)
       {
         my_message(ER_NO_DB_ERROR, ER(ER_NO_DB_ERROR), MYF(0));
         delete lex->sphead;