Fix search addons: searchTerm should not be matched if the beginning of matching index < 0

[ntchjb]

I tested using find previous function in search add-ons. It turns out that searching cannot go to previous line if the current result start at column index 0 as seen in the following picture:

The current selection (the first meow at second line) is stuck and couldn't continue finding other results. I think it's because lastIndexOf function still do matching string at the beginning of searchStringLine although searchStringLine's length is smaller than searchTerm, which should return -1. I fix the code by adding if clause to check whether the comparing range of searchStringLine is shorter than searchTerm or not.

[Tyriar]

Great catch! 😃