Plug a leak in ASN1_item_i2d()

ASN1_item_ex_i2d() does not take ownership of the memory pointed at by
*out, so it's the caller's responsibility to free it on error.

Change-Id: Id8cb70e50f280944418629a32b53fd4ca248b0bd
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/57805
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/asn1/tasn_enc.c

@@ -97,6 +97,7 @@ int ASN1_item_i2d(ASN1_VALUE *val, unsigned char **out, const ASN1_ITEM *it) {
     p = buf;
     int len2 = ASN1_item_ex_i2d(&val, &p, it, /*tag=*/-1, /*aclass=*/0);
     if (len2 <= 0) {
+      OPENSSL_free(buf);
       return len2;
     }
     assert(len == len2);