subpaving assertion violation at ../src/math/subpaving/subpaving_t_def.h Line: 372

Hi,
For this formula:
```
(declare-fun a () Real)
(declare-fun b () Real)
(assert (distinct a (^ b 0.0)))
(check-sat-using subpaving)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/math/subpaving/subpaving_t_def.h
Line: 372
degree(i) > 0
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 3bb05b5
