Fix index expression in interpretert::evaluate

[allredj]

Using integer2string here lead to a value having bad widths.

Sorry for the lack of regression test but I was not able to reproduce this using jbmc. There is however a test-gen regression test that catches the pre-existing bug.

This fixes a bug revealed (but not introduced) by #3100. The bug was only observed on test-gen.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[peterschrammel]

Can you add a reference to the commit which test exposed that? (To the TG commit if it was there).

[allredj]

Spec in models-library:
https://github.com/diffblue/models-library/blob/4d5b06547bda7184179791ec34e8b7585aa5ce7b/model/modelTests/specs/java/util/ArrayList.spec#L217

Test file in models-library:
https://github.com/diffblue/models-library/blob/4d5b06547bda7184179791ec34e8b7585aa5ce7b/model/modelTests/test_data/maven/java/util/ArrayList/src/main/java/L1ToArray_TArray.java#L37

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 46dbbd3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/88897648