TerraSync: fix crashes with null file return

Not sure how this is happening, but, check for a null file object
in FileGetRequest::onDone.

simgear/io/HTTPRepository.cxx

@@ -995,7 +995,10 @@ HTTPRepository::failure() const
       }
 
       void onDone() override {
+          if (file) {
               file->close();
+          }
+
         if (responseCode() == 200) {
           std::string hash =
               strutils::encodeHex(sha1_result(&hashContext), HASH_LENGTH);