[z2045] Fix a bug with SCRIPT_COMMAND_KILL_CREDIT

This bug can happen if a (group) kill credit is provided without involved creature.
Thanks to Atari for pointing!

(based on commit 0fe6964)

Signed-off-by: Nighoo <nighoo@overninethousand.de>

(based on commit [s1658] - ce70a47)