Add missing check in relation inversion logic

[ronenh]

It was adding terms that refer to non-existent realtions.

The following manifest causes an error when calling GetGraph to search for objects that a given subject has access to.

user:
  relations:
    manager: user

resource:
  relations:
    viewer: user#manager

[coveralls]

Pull Request Test Coverage Report for Build 10999738514

Details

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 60.781%

---

Files with Coverage Reduction	New Missed Lines	%
graph/objects.go	2	88.81%

Totals
Change from base Build 10829675514:	-0.04%
Covered Lines:	2816
Relevant Lines:	4633

---

💛 - Coveralls