Add rb_str_locktmp to prevent accidental modifications of z->buf.

Author: ioquatix

ext/zlib/zlib.c

@@ -1053,12 +1053,16 @@ zstream_run_func(struct zstream_run_args *args)
     while (!args->interrupt) {
         n = z->stream.avail_out;
 
+        rb_str_locktmp(z->buf);
+
 #ifndef RB_NOGVL_UBF_ASYNC_SAFE
         err = (int)(VALUE)rb_thread_call_without_gvl(zstream_run_once, (void *)args, zstream_unblock_func, (void *)args);
 #else
         err = (int)(VALUE)rb_nogvl(zstream_run_once, (void *)args, zstream_unblock_func, (void *)args, RB_NOGVL_UBF_ASYNC_SAFE);
 #endif
 
+        // Assuming that `rb_nogvl` doesn't raise an exception, we don't need to deal with `rb_ensure`.
+        rb_str_unlocktmp(z->buf);
         rb_str_set_len(z->buf, ZSTREAM_BUF_FILLED(z) + (n - z->stream.avail_out));
 
         if (err == Z_STREAM_END) {