fix possible unsigned integer underflow

buf_size has to be greater than header_size, otherwise subtracting
header_size from buf_size will cause an integer underflow.

Being equal to header_size is fine, however useless, so quit early.

src/load.c

@@ -673,7 +673,7 @@ mrb_read_irep_file(mrb_state *mrb, FILE* fp)
     goto irep_exit;
   }
   result = read_binary_header(buf, &buf_size, NULL, &flags);
-  if (result != MRB_DUMP_OK) {
+  if (result != MRB_DUMP_OK || buf_size <= header_size) {
     goto irep_exit;
   }