coverity 1462565: Null pointer dereferences

Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11651)

crypto/evp/m_sigver.c

@@ -317,11 +317,13 @@ int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *data, size_t dsize)
                                                       data, dsize);
 
  legacy:
+    if (pctx != NULL) {
         /* do_sigver_init() checked that |digest_custom| is non-NULL */
         if (pctx->flag_call_digest_custom
             && !ctx->pctx->pmeth->digest_custom(ctx->pctx, ctx))
             return 0;
         pctx->flag_call_digest_custom = 0;
+    }
 
     return EVP_DigestUpdate(ctx, data, dsize);
 }