Fix a memory leak in X509_issuer_and_serial_hash

This is reproducible with my error injection patch:

$ ERROR_INJECT=1653267699 ../util/shlib_wrap.sh ./x509-test ./corpora/x509/5f4034ae85d6587dcad4da3e812e80f3d312894d
ERROR_INJECT=1653267699
    #0 0x7fd485a6ad4f in __sanitizer_print_stack_trace ../../../../src/libsanitizer/asan/asan_stack.cc:36
    #1 0x55c12d268724 in my_malloc fuzz/test-corpus.c:114
    #2 0x7fd484f51a75 in CRYPTO_zalloc crypto/mem.c:230
    #3 0x7fd484ed778d in EVP_DigestInit_ex crypto/evp/digest.c:139
    #4 0x7fd4850a9849 in X509_issuer_and_serial_hash crypto/x509/x509_cmp.c:44
    #5 0x55c12d268951 in FuzzerTestOneInput fuzz/x509.c:44
    #6 0x55c12d268239 in testfile fuzz/test-corpus.c:182
    #7 0x55c12d267c7f in main fuzz/test-corpus.c:226
    #8 0x7fd483a42082 in __libc_start_main ../csu/libc-start.c:308
    #9 0x55c12d267e5d in _start (/home/ed/OPCToolboxV5/Source/Core/OpenSSL/openssl/fuzz/x509-test+0x3e5d)

=================================================================
==1058475==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 268 byte(s) in 1 object(s) allocated from:
    #0 0x7fd485a5dc3e in __interceptor_realloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:163
    #1 0x7fd484d2eb9b in BUF_MEM_grow crypto/buffer/buffer.c:97
    #2 0x7fd4850b2913 in X509_NAME_oneline crypto/x509/x509_obj.c:43
    #3 0x7fd4850a982f in X509_issuer_and_serial_hash crypto/x509/x509_cmp.c:41
    #4 0x55c12d268951 in FuzzerTestOneInput fuzz/x509.c:44
    #5 0x55c12d268239 in testfile fuzz/test-corpus.c:182
    #6 0x55c12d267c7f in main fuzz/test-corpus.c:226
    #7 0x7fd483a42082 in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: 268 byte(s) leaked in 1 allocation(s).

crypto/x509/x509_cmp.c

@@ -34,7 +34,7 @@ unsigned long X509_issuer_and_serial_hash(X509 *a)
     unsigned long ret = 0;
     EVP_MD_CTX *ctx = EVP_MD_CTX_new();
     unsigned char md[16];
-    char *f;
+    char *f = NULL;
 
     if (ctx == NULL)
         goto err;
@@ -45,7 +45,6 @@ unsigned long X509_issuer_and_serial_hash(X509 *a)
         goto err;
     if (!EVP_DigestUpdate(ctx, (unsigned char *)f, strlen(f)))
         goto err;
-    OPENSSL_free(f);
     if (!EVP_DigestUpdate
         (ctx, (unsigned char *)a->cert_info.serialNumber.data,
          (unsigned long)a->cert_info.serialNumber.length))
@@ -56,6 +55,7 @@ unsigned long X509_issuer_and_serial_hash(X509 *a)
            ((unsigned long)md[2] << 16L) | ((unsigned long)md[3] << 24L)
         ) & 0xffffffffL;
  err:
+    OPENSSL_free(f);
     EVP_MD_CTX_free(ctx);
     return ret;
 }