[FileNameSearch] Check wasKilled() before searching each file

Check wasKilled() before handling each file in the filenamesearch loop.

Quoting from .../kio/src/core/slavebase.h

If your ioslave was killed by a signal, wasKilled() returns true.
Check it regularly in lengthy functions (e.g. in get();) and return
as fast as possible from this function if wasKilled() returns true.
This will ensure that your slave destructor will be called correctly.

This wasn't happening, the check was done just the once. A file-by-file search can very likely take some time. A check is now done before handling each file.