sftp_write_sliding: send the complete file

When reaching the end of file there can still be data left not sent.

example/sftp_write_sliding.c

@@ -237,6 +237,10 @@ int main(int argc, char *argv[])
         nread = fread(&mem[memuse], 1, sizeof(mem)-memuse, local);
         if (nread <= 0) {
             /* end of file */
+            if (memuse > 0)
+                /* the previous sending is not finished */
+                nread = 0;
+            else
                 break;
         }
         memuse += nread;