fix: segfault when moving scoped_ostream_redirect

[kounelisagis]

Description

The default move constructor of scoped_ostream_redirect caused a segfault when the moved-to object (or the original stream) was subsequently used.

The root cause: = default moved the internal pythonbuf (transferring its d_buffer, pywrite, pyflush and nulling the streambuf base pointers), but left the stream (e.g. std::cout) still pointing at the moved-from buffer. Any subsequent write through the stream called overflow() on the zombie pythonbuf, which dereferenced pptr() (now null) — immediate segfault.

Reproducer

py::scoped_ostream_redirect redir1(std::cout, py::module_::import("sys").attr("stdout"));
std::cout << "before" << std::flush;              // OK
py::scoped_ostream_redirect redir2(std::move(redir1));
std::cout << "after" << std::flush;               // SIGSEGV

Fix

Replace = default with an explicit move constructor that:

1. Moves the buffer to the new location
2. Re-points the stream to the new buffer (costream.rdbuf(&buffer))
3. Sets other.old = nullptr to disarm the moved-from destructor

The destructor is guarded with if (old) so the moved-from object skips restoring the stream.

Suggested changelog entry:

• Fixed segfault when moving scoped_ostream_redirect: the default move constructor left the stream pointing at a moved-from pythonbuf with null internal pointers, causing a null dereference on the next write.