Fix segfault in atom_cluster_noc_group_check()

[soheilshahrouz]

g_vpr_ctx.cl_helper().atom_noc_grp_id is not initialized when try_pack() is not called. This causes a segfault when re-clustering API is used as atom_cluster_noc_group_check() tries to access an uninitialized vector. This PR fixes this problem by check whether the vector is initilzided.

[vaughnbetz]

I suggest a comment saying why this is necessary (some flows that just use the clustering legality check might not have this data structure alive). Can be added after CI completes / in another PR so we get this one in quickly if you like.

[vaughnbetz]

Thanks for the fast action @soheilshahrouz .
Tagging @KA7E as she has a dependency on this one.
Do we have any NoC checks in the fast to run tests (vtr_strong etc.)? This has already passed the fast checks, which is good, but I'm not sure if we have a (tiny) NoC test in those ones. If not it would be good to add one @soheilshahrouz (in another PR) so we have a very fast NoC test.

[vaughnbetz]

Merging since CI passed, but I still think this should have a comment added @soheilshahrouz