Assertion violation on FP formula in /src/util/mpz.cpp Line: 1650

Hi,
For this formula:
```
(declare-fun a () (_ FloatingPoint 65 2))
(assert (= (fp.abs (fp.neg a)) a))
(check-sat)
```
z3 will throw out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/util/mpz.cpp
Line: 1650
is_int64(a)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Revision: d95b549