io.c: add assertions to mrb_io_bufread(); ref #5389

mrbgems/mruby-io/src/io.c

@@ -1476,6 +1476,9 @@ mrb_io_bufread(mrb_state *mrb, mrb_value self)
   mrb_int len;
 
   mrb_get_args(mrb, "Si", &str, &len);
+  mrb_assert(RSTRING_LEN(str) > 0);
+  mrb_assert(RSTRING_PTR(str) != NULL);
+  mrb_str_modify(mrb, RSTRING(str));
   return io_bufread(mrb, str, len);
 }