vg_load_svg: Fix pointing to wrong parent
If defs type is open empty style, it is not included in stack.
Because the current logic is weak to group tags in sibling relationships.
If defs of open-empty type is placed before the group tag,
the group tag refers to the wrong parent. This patch prevents it.
Test Plan: N/A
Reviewers: Hermet, smohanty
Reviewed By: Hermet
Differential Revision: https://phab.enlightenment.org/D12129