readjoiner: fix segfault on non-existant readset

This PR fixes a segfault with readjoiner when a non-existent readset index name is passed to the overlap tool:
```
$ bin/gt readjoiner overlap -readset nonexist -l 23
# gt readjoiner overlap (version 1.2)
zsh: segmentation fault  bin/gt readjoiner overlap -readset nonexist -l 23
```