Fix a memory leak in ec_key_simple_oct2priv

This is reproducible with my error injection patch:

$ ERROR_INJECT=1652710284 ../util/shlib_wrap.sh ./server-test ./corpora/server/4e48da8aecce6b9b58e8e4dbbf0523e6d2dd56dc
140587884632000:error:03078041:bignum routines:bn_expand_internal:malloc failure:crypto/bn/bn_lib.c:282:
140587884632000:error:10103003:elliptic curve routines:ec_key_simple_oct2priv:BN lib:crypto/ec/ec_key.c:662:
140587884632000:error:100DE08E:elliptic curve routines:old_ec_priv_decode:decode error:crypto/ec/ec_ameth.c:464:
140587884632000:error:0D0680A8:asn1 encoding routines:asn1_check_tlen:wrong tag:crypto/asn1/tasn_dec.c:1149:
140587884632000:error:0D07803A:asn1 encoding routines:asn1_item_embed_d2i:nested asn1 error:crypto/asn1/tasn_dec.c:309:Type=X509_ALGOR
140587884632000:error:0D08303A:asn1 encoding routines:asn1_template_noexp_d2i:nested asn1 error:crypto/asn1/tasn_dec.c:646:Field=pkeyalg, Type=PKCS8_PRIV_KEY_INFO
140587884632000:error:0907B00D:PEM routines:PEM_read_bio_PrivateKey:ASN1 lib:crypto/pem/pem_pkey.c:88:

=================================================================
==19676==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x7fdd2a6bb09f in __interceptor_malloc ../../../../gcc-trunk/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x7fdd2a2fa430 in CRYPTO_zalloc crypto/mem.c:230
    #2 0x7fdd2a15df11 in BN_new crypto/bn/bn_lib.c:246
    #3 0x7fdd2a15df88 in BN_secure_new crypto/bn/bn_lib.c:257
    #4 0x7fdd2a247390 in ec_key_simple_oct2priv crypto/ec/ec_key.c:655
    #5 0x7fdd2a241fc5 in d2i_ECPrivateKey crypto/ec/ec_asn1.c:1030
    #6 0x7fdd2a23dac5 in old_ec_priv_decode crypto/ec/ec_ameth.c:463
    #7 0x7fdd2a109db7 in d2i_PrivateKey crypto/asn1/d2i_pr.c:46
    #8 0x7fdd2a33ab16 in PEM_read_bio_PrivateKey crypto/pem/pem_pkey.c:84
    #9 0x7fdd2a3330b6 in PEM_read_bio_ECPrivateKey crypto/pem/pem_all.c:151
    #10 0x402dba in FuzzerTestOneInput fuzz/server.c:592
    #11 0x40370b in testfile fuzz/test-corpus.c:182
    #12 0x402846 in main fuzz/test-corpus.c:226
    #13 0x7fdd297b9f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f44)

SUMMARY: AddressSanitizer: 24 byte(s) leaked in 1 allocation(s).

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from openssl#18361)

crypto/ec/ec_key.c

@@ -657,8 +657,7 @@ int ec_key_simple_oct2priv(EC_KEY *eckey, const unsigned char *buf, size_t len)
         ECerr(EC_F_EC_KEY_SIMPLE_OCT2PRIV, ERR_R_MALLOC_FAILURE);
         return 0;
     }
-    eckey->priv_key = BN_bin2bn(buf, len, eckey->priv_key);
-    if (eckey->priv_key == NULL) {
+    if (BN_bin2bn(buf, len, eckey->priv_key) == NULL) {
         ECerr(EC_F_EC_KEY_SIMPLE_OCT2PRIV, ERR_R_BN_LIB);
         return 0;
     }