Incorrect negative fraction check for quantified permissions

[gauravpartha]

When exhaling a quantified permission, Carbon (in some cases) checks whether the exhaled fraction is strictly greater than 0. However, it should be fine to exhale 0 permission.

field f: Int

predicate P(x: Ref)

method m() {
    label pre_stabilize0
    exhale forall z: Ref :: acc(z.f, none) //this works 
    exhale (forall x: Ref:: acc(P(x), none)) //this fails
}

[gauravpartha]

Duplicate of #171.
Fixed in 3b58d4f.