Fix for error in pan-prediction

Fix for error in pan-prediction

This is to resolve an issue reported by valgrind:

==7892==
==7892== Invalid read of size 4
==7892==    at 0x4B5C7BA: operator-= (vector2.h:196)
==7892==    by 0x4B5C7BA: Dali::Vector2::operator-(Dali::Vector2 const&) const (vector2.h:184)
==7892==    by 0x4B912BF: Dali::Internal::SceneGraph::PanGesture::InterpolatePoint(std::vector<Dali::Internal::SceneGraph::PanGesture::PanInfo, std::allocator<Dali::Internal::SceneGraph::PanGesture::PanInfo> >&, unsigned int, unsigned int, unsigned int, Dali::Internal::SceneGraph::PanGesture::PanInfo&, Dali::Internal::SceneGraph::PanGesture::RelativeVectors&, int, bool) (scene-graph-pan-gesture.cpp:947)

Change-Id: I99e494a5859df175109dc9cf74cf2ad1c7aecbd9
Signed-off-by: Tom Robinson <tom.robinson@samsung.com>