Avoid args-out-of-range errors on fringe clicks after "C-h k"

* src/keyboard.c (echo_truncate): Don't call Ftruncate if the echo
message is already shorter than NCHARS.  (Bug#27040)

src/keyboard.c

@@ -634,7 +634,8 @@ echo_length (void)
 static void
 echo_truncate (ptrdiff_t nchars)
 {
-  if (STRINGP (KVAR (current_kboard, echo_string)))
+  Lisp_Object es = KVAR (current_kboard, echo_string);
+  if (STRINGP (es) && SCHARS (es) > nchars)
     kset_echo_string (current_kboard,
 		      Fsubstring (KVAR (current_kboard, echo_string),
 				  make_number (0), make_number (nchars)));