MINOR: jwt: Make invalid static JWT algorithms an error in `jwt_verify` converter

It is not useful to start a configuration where an invalid static string is
provided as the JWT algorithm. Better make the administrator aware of the
suspected typo by failing to start.
diff --git a/src/sample.c b/src/sample.c
index 9200ca3..5abf471 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -3522,14 +3522,14 @@
 
 		switch(alg) {
 		case JWT_ALG_DEFAULT:
-			memprintf(err, "unknown JWT algorithm : %s", *err);
-			break;
+			memprintf(err, "unknown JWT algorithm: %s", args[0].data.str.area);
+			return 0;
 
 		case JWS_ALG_PS256:
 		case JWS_ALG_PS384:
 		case JWS_ALG_PS512:
 			memprintf(err, "RSASSA-PSS JWS signing not managed yet");
-			break;
+			return 0;
 
 		default:
 			break;