gh-131505: Move len boundary assertions before using len. (#131536)

Move len boundary assertions before using len.

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 4cde5f87032a3fec062485b78263e61226ce488d..61cfec435fe60fb3339a1874c943ce4d837fd615 100644 (file)
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -607,9 +607,9 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
             len = 0;
     }
 
-    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     assert(self->pos + len < PY_SSIZE_T_MAX);
     assert(len >= 0);
+    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     self->pos += len;
 
     return PyLong_FromSsize_t(len);