Fix memory leak if ossl_bn_new() fails

When that call fails, the `bn` BIGNUM is never freed in
asn1integer_to_num(). To solve this, use rb_protect().

Example Valgrind report:
```
32 (24 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 11,113 of 25,910
  malloc (at /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
  CRYPTO_zalloc (at /usr/lib/x86_64-linux-gnu/libcrypto.so.3)
  BN_new (at /usr/lib/x86_64-linux-gnu/libcrypto.so.3)
  BN_bin2bn (at /usr/lib/x86_64-linux-gnu/libcrypto.so.3)
  <unknown stack frame>
 *asn1integer_to_num (ossl_asn1.c:136)
 *asn1integer_to_num_i (ossl_asn1.c:165)
  rb_protect (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
 *decode_int (ossl_asn1.c:356)
 *int_ossl_asn1_decode0_prim (ossl_asn1.c:777)
 *ossl_asn1_decode0 (ossl_asn1.c:936)
 *ossl_asn1_decode_all (ossl_asn1.c:1058)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  rb_catch_obj (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_yield (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_ary_each (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  rb_catch_obj (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  <unknown stack frame>
  <unknown stack frame>
  <unknown stack frame>
  rb_vm_exec (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_vm_invoke_proc (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
  rb_proc_call_kw (at /usr/lib/x86_64-linux-gnu/libruby-3.2.so.3.2.3)
```

Author: ndossche

ext/openssl/ossl_asn1.c

@@ -130,15 +130,17 @@ asn1integer_to_num(const ASN1_INTEGER *ai)
     if (!ai) {
         ossl_raise(rb_eTypeError, "ASN1_INTEGER is NULL!");
     }
+
+    num = ossl_bn_new(BN_value_one());
+    bn = GetBNPtr(num);
+
     if (ASN1_STRING_type(ai) == V_ASN1_ENUMERATED)
-        bn = ASN1_ENUMERATED_to_BN(ai, NULL);
+        bn = ASN1_ENUMERATED_to_BN(ai, bn);
     else
-        bn = ASN1_INTEGER_to_BN(ai, NULL);
+        bn = ASN1_INTEGER_to_BN(ai, bn);
 
     if (!bn)
         ossl_raise(eOSSLError, NULL);
-    num = ossl_bn_new(bn);
-    BN_free(bn);
 
     return num;
 }