Clear Rsr back-pointers when Statement relinquishes ownership

[dand-oss]

When forced statement cleanup encounters an invalid attachment handle, Statement::release() can destroy the Statement while its Rsr remains alive. Rsr::rsr_self then points into the freed Statement; the later Rsr destructor reads that freed member. Clear the back-pointer when Statement relinquishes its handle and in the Statement destructor when a handle remains.

Independently reproduced on Linux x86-64 using the Firebird 5 ASan/UBSan debug build with system allocation enabled. A focused test compiled the actual remote/client/interface.cpp and linked the existing production client objects and libraries. The before/after sources differed only by the two fix hunks, with the same test entry point appended:

Rsr* handle = FB_NEW Rsr;
Remote::Statement* statement = FB_NEW Remote::Statement(handle, nullptr, 3);
statement->addRef();
statement->release();
delete handle;

With the fix removed, ASan reports a heap-use-after-free in Rsr::~Rsr(): an 8-byte read from the Statement allocation freed by Remote::Statement::release(). With the fix applied, the same test exits 0 with no ASan/UBSan report. ASan used detect_leaks=0 and halt_on_error=1.

The current-master patch also passes GCC 16.2 C++20 syntax compilation and git diff --check. The runtime test exercises the production ownership and forced-cleanup path through fault injection; it is not an end-to-end WinGLUE session or a runtime test of master.