[3.14] gh-148286: Fix UB in ZstdDecompressor.unused_data when a frame is decompressed in one call (GH-153258) (#153645)

(cherry picked from commit adebb68)

Co-authored-by: Emma Smith <emma@emmatyping.dev>

Author: StanFromIreland

Misc/NEWS.d/next/Library/2026-07-07-13-31-52.gh-issue-148286.-qu-em.rst

@@ -0,0 +1,3 @@
+Fix undefined behavior in
+:attr:`compression.zstd.ZstdDecompressor.unused_data` when a complete frame
+was decompressed in a single call.

Modules/_zstd/decompressor.c

@@ -594,9 +594,14 @@ _zstd_ZstdDecompressor_unused_data_get_impl(ZstdDecompressor *self)
     }
     else {
         if (self->unused_data == NULL) {
-            self->unused_data = PyBytes_FromStringAndSize(
-                                    self->input_buffer + self->in_begin,
-                                    self->in_end - self->in_begin);
+            if (self->input_buffer == NULL) {
+                self->unused_data = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+            }
+            else {
+                self->unused_data = PyBytes_FromStringAndSize(
+                                        self->input_buffer + self->in_begin,
+                                        self->in_end - self->in_begin);
+            }
             ret = self->unused_data;
             Py_XINCREF(ret);
         }