Check whether credentials are missing before using them

Signed-off-by: Waldemar Quevedo <wally@synadia.com>

nats.go

@@ -1051,6 +1051,12 @@ func UserJWT(userCB UserJWTHandler, sigCB SignatureHandler) Option {
 		if sigCB == nil {
 			return ErrUserButNoSigCB
 		}
+		// Smoke test the user callback to ensure it is setup properly
+		// when processing options.
+		if _, err := userCB(); err != nil {
+			return err
+		}
+
 		o.UserJWT = userCB
 		o.SignatureCB = sigCB
 		return nil