tls1_set_groups_list: freeing *pext before overwriting

calling SSL_CTX_set1_groups_list() twice on one SSL_CTX* caused a memory
leak visible in valgrind:
  4 bytes in 1 blocks are definitely lost in loss record 1 of 1
     at 0x4841888: malloc (vg_replace_malloc.c:381)
     by 0x4B1EE96: CRYPTO_memdup (in libcrypto.so.3)
     by 0x48993A0: tls1_set_groups_list (in libssl.so.3)
     by 0x487AA7E: ssl3_ctx_ctrl (in libssl.so.3)
     by 0x1091EA: main (mem_leak.c:10)

  LEAK SUMMARY:
     definitely lost: 4 bytes in 1 blocks

Freeing *pext to fix it.

CLA: trivial

Signed-off-by: Peter Kaestle <peter.kaestle@nokia.com>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20317)

(cherry picked from commit fcf3a9f7)