Openssh: double free in red hat enterprise linux versions of openssh dh-gex client path during fips known-group validation leads to client-side denial of service

An attacker-controlled SSH server negotiates `diffie-hellman-group-exchange-sha256` with an affected client and returns DH group parameters `(p, g)` that pass the size check but fail the FIPS known-group validation. The client's `input_kex_dh_gex_group()` function transfers ownership of those BIGNUMs into `kex->dh` via `dh_new_group()`, then jumps to `out` on the validation failure without nulling the local pointers. The local cleanup frees the BIGNUMs, and later teardown in `kex_free()` calls `DH_free(kex->dh)`, which frees the same objects again, causing a double free [ref_id=1].

The vulnerability is in `kexgexc.c` in the `input_kex_dh_gex_group()` function, with later teardown in `kex.c` (`kex_free()`). The bug occurs because the pointer nullification (`p = g = NULL`) is placed *after* the FIPS known-group check, so when that check fails and execution jumps to `out`, the local `BN_clear_free()` calls free BIGNUMs that are already owned by `kex->dh`, and a subsequent `DH_free(kex->dh)` frees them again.

The proposed patch moves the `p = g = NULL` assignment immediately after the successful `dh_new_group()` call, before the FIPS known-group check. This ensures that when the validation fails and execution jumps to `out`, the local pointers are already null, so the `BN_clear_free()` calls are no-ops and the BIGNUMs remain owned solely by `kex->dh` for proper cleanup via `DH_free()` [ref_id=1].