Fix error "uninitialized value in subtraction"

checks element of $LAST_MATCH_START and $LAST_MATCH_END are defined.

Error message: 

```
Use of uninitialized value in subtraction (-) at lib/perl5/Slack/App.pm line 214.
Use of uninitialized value in substr at lib/perl5/Slack/App.pm line 214.
````