lra trace Assertion violation at ../src/util/old_vector.h Line: 375

Hi,
For this formula:
```
(set-option :trace true)
(declare-fun a () Float32)
(assert (= a ((_ to_fp 8 24) roundTowardZero a)))
(check-sat-using lra)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/util/old_vector.h
Line: 375
idx < size()
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 5516e42
