bpo-35714: Reject null characters in struct format strings (GH-16928)

struct.error is now raised if there is a null character in a struct
format string.
(cherry picked from commit 3f59b55316)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Modules/_struct.c

@@ -1296,6 +1296,11 @@ prepare_s(PyStructObject *self)
     size_t ncodes;
 
     fmt = PyBytes_AS_STRING(self->s_format);
+    if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(self->s_format)) {
+        PyErr_SetString(_structmodulestate_global->StructError,
+                        "embedded null character");
+        return -1;
+    }
 
     f = whichtable(&fmt);