EVP_PKEY_asn1_add0(): Check that this method isn't already registered

No two public key ASN.1 methods with the same pkey_id can be
registered at the same time.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/4596)

crypto/asn1/ameth_lib.c

diff --git a/crypto/asn1/ameth_lib.c b/crypto/asn1/ameth_lib.c
index 5f523b2..87a3eba 100644
--- a/crypto/asn1/ameth_lib.c
+++ b/crypto/asn1/ameth_lib.c
@@ -137,6 +137,11 @@
 
 int EVP_PKEY_asn1_add0(const EVP_PKEY_ASN1_METHOD *ameth)
 {
+    if (pkey_asn1_find(ameth->pkey_id) != NULL) {
+        EVPerr(EVP_F_EVP_PKEY_ASN1_ADD0,
+               EVP_R_PKEY_ASN1_METHOD_ALREADY_REGISTERED);
+        return 0;
+    }
     if (app_methods == NULL) {
         app_methods = sk_EVP_PKEY_ASN1_METHOD_new(ameth_cmp);
         if (app_methods == NULL)