[crypto] Fix memory leak in cms_verify_digest()

Detected using Valgrind.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/cms.c

 				    info->signature_len ) ) != 0 ) {
 		DBGC ( sig, "CMS %p/%p signature verification failed: %s\n",
 		       sig, info, strerror ( rc ) );
-		return rc;
+		goto err_verify;
 	}
 
+ err_verify:
 	pubkey_final ( pubkey, ctx );
  err_init:
 	return rc;