Fix notice: check if array key exists before the value gets returned in PHP/CompatInfo/Filter.php

This patch fixes a notice, when running `phpcompatinfo -vv print -R --reference ALL --report function --filter-version 5.2.6 ./Zend/` against Zend Framewoek 1.12.3

```
Undefined index: 2 in PHP/CompatInfo/Filter.php on line 69
```