CVE-2026-64404: Bluetooth: ISO: avoid NULL deref of conn in iso_conn_big_sync()

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: ISO: avoid NULL deref of conn in isoconnbigsync()

isoconnbigsync() drops the socket lock to call hcigetroute() and then re-acquires it, but dereferences isopi(sk)->conn->hcon afterwards without re-checking that conn is still valid.

While the lock is dropped, the connection can be torn down under the same socket lock: isodisconncfm() -> isoconndel() -> isochandel() sets isopi(sk)->conn to NULL (and the broadcast teardown path can also clear conn->hcon on its own). When isoconnbigsync() re-acquires the lock and reads conn->hcon, conn may be NULL, causing a NULL pointer dereference (hcon is the first member of struct isoconn).

This path is reached from isosockrecvmsg() for a PA-sync broadcast sink socket (BTSKDEFERSETUP | BTSKPASYNC), so the dropped-lock window can race with connection teardown driven by controller events.

Re-validate isopi(sk)->conn and its hcon after re-acquiring the socket lock and bail out if the connection went away, as already done in the sibling isosockrebindbc().