Concolic Fuzzer -- possible bug in the Zint wrapper

[vrthra]

Reported by Hila Peleg:

I'm going through the concolic testing tutorial in the debugging book and running it on some examples, and I've come across a possible bug in the zint wrapper:

Line 1527 in ConcolicFuzzer.py is

cz = z3.SubString(self.z, last_idx.z, 1)

but I keep stumbling on scenarios where the computation of last_idx in the line above returns a concrete int rather than a zint, and so last_idx.z crashes.