[ticket/11437] avoid $start becoming negative when no search results are found

When search returns no results there is no need to go inside the if
statement. Since $result_count becomes zero, $start becomes negative
which leads to failed assertion.

PHPBB3-11437
Merge remote-tracking branch 'dhruvgoel92/ticket/11437' into develop

* dhruvgoel92/ticket/11437:
  [ticket/11437] avoid $start becoming negative when no search results are found