fe-dcc-(get|send): Fix some -Wpointer-compare with newer gcc

The warning itself:

>warning: comparison between pointer and zero character constant [-Wpointer-compare]

Harmless stuff as far as I can tell.

The fix adds a null check that probably isn't needed. The old code that
compared against '\0' worked a lot like a null check so it makes sense
to keep that, while also adding the intended check for empty string.

This was visible with "/dcc close send a" showing an empty filename.
The equivalent for get didn't show the filename in the format string.

0 files changed, 0 insertions, 0 deletions