Makes mrb_any_to_s() accept an object whose class is NULL

When using `mrb_any_to_s()` for debugging purposes, giving an object
whose class is `NULL` no longer causes a SIGSEGV and no crash.
This is achieved by making `mrb_class_name()` and `mrb_str_cat_cstr()`
null safe.

src/string.c

@@ -2759,7 +2759,7 @@ mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len)
 MRB_API mrb_value
 mrb_str_cat_cstr(mrb_state *mrb, mrb_value str, const char *ptr)
 {
-  return mrb_str_cat(mrb, str, ptr, strlen(ptr));
+  return mrb_str_cat(mrb, str, ptr, ptr ? strlen(ptr) : 0);
 }
 
 MRB_API mrb_value