Replaced rand() with mt_rand()

mt_rand — Generate a better random value.

Excuse me, I laughed for quite a bit after reading this. Fixes #2
Prevent possible giveaway closure

Someone can POST any 'name' at giveaway.php with a given ID to close it,
even without dispensing the key. Fixes #1
Prevent possible giveaway closure (2)

Sorry, I broke it. Thanks, @YePpHa
Merge pull request #3 from DragoonAethis/master

Replaced rand() with mt_rand() + prevent possible giveaway closure