Check transfer file isnt null before trying to delete it

Fixes issue 3685

Change-Id: I4c2f504d87b6128c6c92a697592bf34061196863
Reviewed-on: http://gerrit.dmdirc.com/749
Automatic-Compile: Gregory Holmes <greg@dmdirc.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/addons/dcc/DCCSend.java

             handler.socketClosed(this);
         }
         // Try to delete empty files.
-        if (transferType == TransferType.RECEIVE && transferFile.length() == 0) {
+        if (transferType == TransferType.RECEIVE && transferFile != null && transferFile.length() == 0) {
             transferFile.delete();
         }
         synchronized (SENDS) {