lib: parser: keep subgraph pointers on stack

lib: parser: keep subgraph pointers on stack

There's no need to malloc() these, we can just keep them on the stack.
The entire struct will get copied around, but replacing a 2-pointer copy
with a 1-pointer copy + malloc + free overhead is not quite efficient.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>