ENGINE_pkey_asn1_find_str(): don't assume an engine implements ASN1 method

Just because an engine implements algorithm methods, that doesn't mean
it also implements the ASN1 method.  Therefore, be careful when looking
for an ASN1 method among all engines, don't try to use one that doesn't
exist.

Fixes #6381
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6383)

(cherry picked from commit 1ac3cd6277f880fac4df313702d5e3b3814e56e2)
(cherry picked from commit 13b578ada3106a4ce9b836f167ee520539f5fa8f)

crypto/engine/tb_asnmth.c

@@ -171,7 +171,8 @@ static void look_str_cb(int nid, STACK_OF(ENGINE) *sk, ENGINE *def, void *arg)
         ENGINE *e = sk_ENGINE_value(sk, i);
         EVP_PKEY_ASN1_METHOD *ameth;
         e->pkey_asn1_meths(e, &ameth, NULL, nid);
-        if (((int)strlen(ameth->pem_str) == lk->len)
+        if (ameth != NULL
+                && ((int)strlen(ameth->pem_str) == lk->len)
                 && strncasecmp(ameth->pem_str, lk->str, lk->len) == 0) {
             lk->e = e;
             lk->ameth = ameth;