check return value from get_core_update() before foreach()

[lybica]

Warning: Invalid argument supplied for foreach() in {...}/wp-admin/update-core.php on line 113

foreach() does not like the "false" returned from get_core_update() (in wp-admin/includes/update.php).
functions in update-core.php should perform return value checks like:

  ...
  if( is_array($updates) ) {
  foreach( $updates as $update ) {
  ...

(dismissed_updates(), core_upgrade_preamble() in wp-admin/update-core.php)

[ryan]

(In [9698]) Cast to array. fixes #8148