Remove a dead check for errmsg in handle_control_authenticate

Coverity doesn't like doing NULL checks on things that can't be
NULL; I like checking things where the logic for their not being
NULL is nontrivial.  Let's compromise, and make it obvious that this
field can't be NULL.

[Coverity CID 202004]

src/or/control.c

@@ -1039,7 +1039,7 @@ handle_control_authenticate(control_connection_t *conn, uint32_t len,
 {
   int used_quoted_string = 0;
   const or_options_t *options = get_options();
-  const char *errstr = NULL;
+  const char *errstr = "Unknown error";
   char *password;
   size_t password_len;
   const char *cp;
@@ -1199,8 +1199,7 @@ handle_control_authenticate(control_connection_t *conn, uint32_t len,
 
  err:
   tor_free(password);
-  connection_printf_to_buf(conn, "515 Authentication failed: %s\r\n",
-                           errstr ? errstr : "Unknown reason.");
+  connection_printf_to_buf(conn, "515 Authentication failed: %s\r\n", errstr);
   connection_mark_for_close(TO_CONN(conn));
   return 0;
  ok: