drush_make_updatexml does not catch when release history xml was not found

drush_make_updatexml does not catch when release history xml was not found because release history is not NULL

as an an example when one has a make file like

core = 6.x
projects[] = content

then release_history is

object(SimpleXMLElement)#2 (1) {
  [0]=>
  string(65) "No release history was found for the requested project (content)."
}

as the call to load the xml succeeded but there is no project "content"
I will post a simple patch that just checks for the existence of $release_history->name instead of release_history, but maybe these should be different checks with different errors?

also the for each in the same function is not protected so I am thinking there needs to be a check for $release_history->terms->term as well