A compacted version that does not load the globals yet calls upon them directly.
if(
is_user_logged_in() and // Force user validation... STUFF
!empty($GLOBALS['post']) and // Check if a $post exists in globals
!empty($GLOBALS['current_user']) and // Check if current_user exists in globals
($GLOBALS['current_user']->ID == $GLOBALS['post']->post_author) // Match User IDs
){
// This is your Author speaking, use the Force!
}
Regards.