Fix ICE with associative arrays

[vindexbit]

This simple example could not compile with the -frelease flag:

string[string] aarr = ["name": "John", "family": "Doe"];
void main() {}

Error:

internal compiler error: Segmentation fault
    1 | string[string] aarr = ["name": "John", "family": "Doe"];

The problem was in checking the borders.