Fix FCLoopIntegralToGraph for explicit kinematics

This works 
```
FCClearScalarProducts[]
FCLoopIntegralToGraph[SFAD[{k, m^2}, k - p, k - pb], {k}]
```
while this doesn't

```
FCClearScalarProducts[]
SPD[p] = m^2
SPD[pb] = m^2
SPD[p, pb] = s/2 - m^2;
FCLoopIntegralToGraph[SFAD[{k, m^2}, k - p, k - pb], {k}]
```

Which is not how it is supposed to be. The detection of tadpoles should be severly improved ...