engine: guard against multiple calls to engine_pka_destroy

[xnox]

When engine is missused, engine_pka_destroy may be called multiple
times. When this happens, memory is freed which no longer was
allocated by the engine, leading to stack corruption and sigaborts.

Whilst engines should not be missused like that, it's best to guard
against such scenarios. It is not the first engine that has been
tripped by other software missusing openssl engine api's.

Sample gdb debuggin session with this patch applied:

    $ gdb curl
    (gdb) break engine_pka_destroy
    Function "engine_pka_destroy" not defined.
    Make breakpoint pending on future shared library load? (y or [n]) y
    Breakpoint 1 (engine_pka_destroy) pending.
    (gdb) run -O https://tpo.pe/pathogen.vim
    Thread 1 "curl" hit Breakpoint 1, engine_pka_destroy (e=0xaaaaaab59010) at e_bluefield.c:594
    warning: Source file is more recent than executable.
    594	    if (pka_rsa_meth) {
    (gdb) p pka_rsa_meth
    $1 = (RSA_METHOD *) 0xaaaaaab58d40
    (gdb) p pka_rsa_meth*
    A syntax error in expression, near `'.
    (gdb) p *pka_rsa_meth
    $2 = {name = 0xaaaaaab592c0 "BlueField RSA method",
      rsa_pub_enc = 0xfffff7a8c3a0 <rsa_ossl_public_encrypt>,
      rsa_pub_dec = 0xfffff7a8c040 <rsa_ossl_public_decrypt>,
      rsa_priv_enc = 0xfffff7a8cbb0 <rsa_ossl_private_encrypt>,
      rsa_priv_dec = 0xfffff7a8c728 <rsa_ossl_private_decrypt>,
      rsa_mod_exp = 0xfffff6af78c0 <engine_pka_rsa_mod_exp>,
      bn_mod_exp = 0xfffff6af76f0 <engine_pka_bn_mod_exp>, init = 0x0, finish = 0x0, flags = 0,
      app_data = 0x0, rsa_sign = 0x0, rsa_verify = 0x0, rsa_keygen = 0x0,
      rsa_multi_prime_keygen = 0x0}
    (gdb) n
    595		RSA_meth_free(pka_rsa_meth);
    (gdb) n
    596		pka_rsa_meth = NULL;
    (gdb) n
    600	    if (pka_dsa_meth) {
    (gdb) p pka_dsa_meth
    $3 = (DSA_METHOD *) 0xaaaaaab5e360
    (gdb) p *pka_dsa_meth
    $4 = {name = 0xaaaaaab592e0 "BlueField DSA method",
      dsa_do_sign = 0xfffff79f58a0 <dsa_do_sign>,
      dsa_sign_setup = 0xfffff79f5890 <dsa_sign_setup_no_digest>,
      dsa_do_verify = 0xfffff79f5190 <dsa_do_verify>,
      dsa_mod_exp = 0xfffff6af77f8 <engine_pka_dsa_mod_exp>,
      bn_mod_exp = 0xfffff6af8a40 <engine_pka_dsa_bn_mod_exp>, init = 0x0, finish = 0x0,
      flags = 0, app_data = 0x0, dsa_paramgen = 0x0, dsa_keygen = 0x0}
    (gdb) n
    601		    DSA_meth_free(pka_dsa_meth);
    (gdb) n
    602		pka_dsa_meth = NULL;
    (gdb) n
    606	    if (pka_dh_meth) {
    (gdb) p pka_dh_meth
    $5 = (DH_METHOD *) 0xaaaaaab53a90
    (gdb) p *pk_dh_meth
    No symbol "pk_dh_meth" in current context.
    (gdb) p *pka_dh_meth
    $6 = {name = 0xaaaaaab59300 "BlueField DH method",
      generate_key = 0xfffff79f02f8 <generate_key>, compute_key = 0xfffff79f00f8 <compute_key>,
      bn_mod_exp = 0xfffff6af77d8 <engine_pka_dh_bn_mod_exp>, init = 0x0, finish = 0x0,
      flags = 0, app_data = 0x0, generate_params = 0x0}
    (gdb) n
    607		    DH_meth_free(pka_dh_meth);
    (gdb) n
    608		pka_dh_meth = NULL;
    (gdb) p *pka_dh_meth
    $7 = {name = 0x0, generate_key = 0xaaaaaaaf3010,
      compute_key = 0xfffff79f00f8 <compute_key>,
      bn_mod_exp = 0xfffff6af77d8 <engine_pka_dh_bn_mod_exp>, init = 0x0, finish = 0x0,
      flags = 0, app_data = 0x0, generate_params = 0x0}
    (gdb) p pka_dh_meth
    $8 = (DH_METHOD *) 0xaaaaaab53a90
    (gdb) n
    611	if (pka_ec_key_meth) {
    (gdb) p pka_dh_meth
    $9 = (DH_METHOD *) 0x0
    (gdb) n
    612	    EC_KEY_METHOD_free(pka_ec_key_meth);
    (gdb) n
    613	    pka_ec_key_meth = NULL;
    (gdb) n
    618	    return 1;
    (gdb) continue
    Continuing.
    PKA_ENGINE: PKA instance is invalid
    PKA_ENGINE: failed to retrieve valid instance
    PKA_ENGINE: PKA instance is invalid
    PKA_ENGINE: failed to retrieve valid instance
    PKA_ENGINE: PKA instance is invalid
    PKA_ENGINE: failed to retrieve valid instance
    PKA_ENGINE: PKA instance is invalid
    PKA_ENGINE: failed to retrieve valid instance
    100   338  100   338    0     0      3      0  0:01:52  0:01:28  0:00:24     3

    Thread 1 "curl" hit Breakpoint 1, engine_pka_destroy (e=0xaaaaaab24320) at e_bluefield.c:594
    594	    if (pka_rsa_meth) {
    (gdb) p pka_rsa_meth
    $10 = (RSA_METHOD *) 0x0
    (gdb) n
    600	    if (pka_dsa_meth) {
    (gdb) p pka_dsa_meth
    $11 = (DSA_METHOD *) 0x0
    (gdb) n
    606	    if (pka_dh_meth) {
    (gdb) p pka_dh_meth
    $12 = (DH_METHOD *) 0x0
    (gdb) n
    611	if (pka_ec_key_meth) {
    (gdb) p pka_ec_key_meth
    $13 = (EC_KEY_METHOD *) 0x0
    (gdb) n
    618	    return 1;
    (gdb) continue
    Continuing.
    warning: Temporarily disabling breakpoints for unloaded shared library "/usr/lib/aarch64-linux-gnu/engines-1.1/pka.so"
    [Inferior 1 (process 73661) exited normally]

[kblaiech]

Thanks!