[3.14] gh-134768: Fix definition of mt_continue_should_break() (GH-134769) (#134916)

gh-134768: Fix definition of `mt_continue_should_break()` (GH-134769)

In 121ed71, mt_continue_should_break
was changed to be guarded by `Py_DEBUG`, but it's used in `compress_mt_continue_lock_held`
with just `assert`, so it needs to be available when `NDEBUG` is undefined
too.

`Py_DEBUG` implies `NDEBUG` is undefined, so we can check just that.
(cherry picked from commit 2f2bee2)


Fixes: 121ed71

Co-authored-by: Sam James <sam@gentoo.org>

Author: miss-islington

Modules/_zstd/compressor.c

@@ -512,7 +512,7 @@ compress_lock_held(ZstdCompressor *self, Py_buffer *data,
     return NULL;
 }
 
-#ifdef Py_DEBUG
+#ifndef NDEBUG
 static inline int
 mt_continue_should_break(ZSTD_inBuffer *in, ZSTD_outBuffer *out)
 {