Fixed an error with the '+=' operator on string variables

When attempting to concatenate and assign to a previously empty string
with +=, it'd throw a memory manager error. Follow-up to 22d2718.

Signed-off-by: Haru <haru@dotalux.com>
Update warper.txt

Bug Fix
- http://hercules.ws/board/tracker/issue-7671-warper-npc-bug-for-pre-re/?gopid=20248#entry20248

* For a end, the script will terminate, but returns nothing, because there's nothing else to be returned, as with a close, the script closes and ends the above function.
Merge branch 'master' of https://github.com/HerculesWS/Hercules into ghbb