[3.14] gh-131505: Move len boundary assertions before using len. (GH-131536) (GH-134239)

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

Move len boundary assertions before using len.
(cherry picked from commit c45e661)

Co-authored-by: naya451 <[email protected]>

Author: miss-islington

Modules/_io/bytesio.c

@@ -583,9 +583,9 @@ _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);