Prevent NativeVote menu callback from freeing in-use forward handle

[nosoop]

Quick-fix while alliedmodders/sourcemod#1041 gets sorted out.

SourceMod doesn't seem to take kindly to a forward handle being deleted while it's being invoked, which is what Handler_NV_MapVoteMenu does with NativeVote.Clear() in nativevotes_mapchooser.sp. This seemingly didn't have disastrous consequences until Debian 10.

All this does is ensure that the handle remains valid for the duration of the private forward call.

[Naleksuh]

@powerlord pls