gesture-action: fix memory corruption

abcf1d589f29ba7914d5648bb9814ad26c13cd83 introduced a crasher because
the 'point' variable points to a piece of memory that is being
reallocated by the begin_gesture (by a g_array_set_size) call 5 lines
before.

https://bugzilla.gnome.org/show_bug.cgi?id=710227