fixes for valgrind failures

sql/item.cc:
  don't forget to adjust the length of the string when removing leading spaces
sql/sql_acl.cc:
  when updating the hostname of the ACL_USER, update the hostname_length too
sql/sql_parse.cc:
  first compare the username string, then test the host pointer
  (host pointer is undefined when the username string is one of the hard-coded values
  set by the parser). This is not a bug, old code is perfectly safe as the undefined
  host pointer is never dereferenced, but let's keep valgrind happy.

sql/item.cc

@@ -1078,6 +1078,7 @@ void Item::set_name(const char *str, uint length, CHARSET_INFO *cs)
   if (!cs->ctype || cs->mbminlen > 1)
   {
     str+= cs->cset->scan(cs, str, str + length, MY_SEQ_SPACES);
+    length-= str - str_start;
   }
   else
   {

sql/sql_acl.cc

@@ -8868,6 +8868,7 @@ static int handle_grant_struct(enum enum_acl_lists struct_no, bool drop,
         acl_user->user.str= strdup_root(&acl_memroot, user_to->user.str);
         acl_user->user.length= user_to->user.length;
         acl_user->host.hostname= strdup_root(&acl_memroot, user_to->host.str);
+        acl_user->hostname_length= user_to->host.length;
         break;
 
       case DB_ACL:

sql/sql_parse.cc

@@ -4335,12 +4335,12 @@ case SQLCOM_PREPARE:
   case SQLCOM_SHOW_GRANTS:
   {
     LEX_USER *grant_user= lex->grant_user;
+    Security_context *sctx= thd->security_ctx;
     if (!grant_user)
       goto error;
 
-    if (grant_user->user.str && grant_user->host.str &&
-        !strcmp(thd->security_ctx->priv_user, grant_user->user.str) &&
-        !strcmp(thd->security_ctx->priv_host, grant_user->host.str))
+    if (grant_user->user.str && !strcmp(sctx->priv_user, grant_user->user.str) &&
+        grant_user->host.str && !strcmp(sctx->priv_host, grant_user->host.str))
       grant_user->user= current_user;
 
     if (grant_user->user.str == current_user.str ||