Minor fix on BG script.

Based on this report.
http://rathena.org/board/tracker/issue-8813-minor-warning-on-battleground-script/
Merge pull request #63 from sandbox-ra/patch-7

Minor fix on BG script. (bugreport:8813)
http://rathena.org/board/tracker/issue-8813-minor-warning-on-battleground-script/