Z3 assertion violation on String formula with str.at, push in ../src/util/old_vector.h line 375

Hi,
For this formula,
```
(set-logic QF_S)
(declare-fun s () String)
(assert (= (str.at s 2) ""))
(push 1)
(check-sat)
```
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
Revision: 23fcc21