client.c: allow empty user names

Zero-length strings passed to -u are valid user names for DTLS.
The function cmdline_read_user() therefore returns 0 when the
argument passed with option '-u' is an empty string.

examples/client.c

@@ -1023,9 +1023,8 @@ cmdline_read_user(char *arg, unsigned char *buf, size_t maxlen) {
   size_t len = strnlen(arg, maxlen);
   if (len) {
     memcpy(buf, arg, len);
-    return len;
   }
-  return -1;
+  return len;
 }
 
 static ssize_t