Bug#11764633 : 57491: THD->MAIN_DA.IS_OK() ASSERT IN EMBEDDED

Issue:
While running embedded server, if client issues TEE command (\T foo/bar) and
"foo/bar" directory doesn't exist, it is suppose to give error. But it was
aborting.  This was happening because wrong error handler was being called.

Solution:
Modified calls to correct error handler. In embedded server case, there are 
two error handler (client and server) which are supposed to be called based 
on which context code is in. If it is in client context, client error handler
should be called otherwise server.

Test case:
Test case automation is not possible as current (following) code doesn't 
allow '\T' to be executed from command line (OR command read from a file):
[client/mysql.cc]
...
 static int
 com_tee(String *buffer __attribute__((unused)),
         char *line __attribute__((unused)))
 {
   char file_name[FN_REFLEN], *end, *param;

   if (status.batch) << THIS IS TRUE WHILE EXECUTING FROM COMMAND LINE.
      return 0;
 ...
So, not adding test case in GA. WIll add a test case in mysql-trunk after 
removing above code so that this could be properly tested before GA.

sql/sql_class.cc

@@ -1196,6 +1196,25 @@ bool THD::store_globals()
   return 0;
 }
 
+/*
+  Remove the thread specific info (THD and mem_root pointer) stored during
+  store_global call for this thread.
+*/
+bool THD::restore_globals()
+{
+  /*
+    Assert that thread_stack is initialized: it's necessary to be able
+    to track stack overrun.
+  */
+  DBUG_ASSERT(thread_stack);
+  
+  /* Undocking the thread specific data. */
+  my_pthread_setspecific_ptr(THR_THD, NULL);
+  my_pthread_setspecific_ptr(THR_MALLOC, NULL);
+  
+  return 0;
+}
+
 
 /*
   Cleanup after query.