Fix FlxObject#(overlaps|overlapsAt) checking only first element of FlxGroup

FlxObject#overlaps and FlxObject#overlapsAt had a bug that would lead to only the first element of a FlxGroup being checked. It was happening because the variable "length" was not locally declared, however there was no compiling warns/errors because the Object class has a property named "length".

Fixed FlixelCommunity/flixel#9, AdamAtomic/flixel#223
Fixed FlixelCommunity/flixel#54, AdamAtomic/flixel#177
Fixed FlixelCommunity/flixel#39, AdamAtomic/flixel#192