Refactor: libpacemaker: Check whether instance already assigned

...before early assignment, and skip if so.

This was turned into a CRM_CHECK() by commit 454d09e6. However, while
working on some other fixes, I hit the CRM_CHECK() in several tests.
It's safer to just check the pe_rsc_provisional flag. We can do this
outside of assign_instance(), so that we don't call preferred_node().

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>